What I use all the time and keep buying more of as these wear out:
A brush with a handle that will fit in the red part of the waterer I've chosen. Has loop for hanging on hook on coop.
A jug of soap that sits outside near coop. (for washing waterer and feeder)
Gloves. Gloves. Gloves.
Willow basket big enough to take out kitchen scraps for the chickens and also the compost and hold my gloves as well as the eggs I collect. Meaning that I have a large basket that will hold two yogurt containers. One yogurt container holds chicken scraps; the other holds the compost bits. It's really handy to have a basket that big. And the gloves, too, for handling chicken stuff or pulling weeds in garden.
Pine shavings for the nest boxes. Keeps eggs really clean.
DE (food grade)
Cloth clothesline for hanging feeders. Some use chains.
Simple stainless carabiners. These are indispensable for hanging feeders and other chicken stuff
Bungie cords for securing doors in the open position (wind issues) or holding doors shut once I go into the coop. I also use them to hold the nesting boxes shut at night. And also they get used to hold on any tarps I may need to put up on ark, etc., for shade or temp rain protection.
What I also use:
Metal garbage can for storing feed. Not plastic because mice can gnaw through it.
Bathroom caulking in a plastic squeeze tube to fill all seems on the metal garbage can to keep all moisture out.
Scoop for the feed. We have one large aluminum one and then some 3 cup measuring ones with handles.
Black rubber tub (about 5" tall and 18" in diameter) (for free range feeding) that I put the feeders in to catch all the feed that the chickens spill. When they empty the feeder, they then get to scratch around in the black tub to clean up the rest. Cost savings and keeps chickens from digging through their own muck to pick up the spilled feed.
Small broom or dustpan for sweeping up small problems in the coop or run. Gets mucky, but it washes.
Sprayer to put on end of hose for rinsing the grass after moving the chicken ark or tractor.
Cinder blocks for setting the waterer on during the day when I move it out and away from coop to keep out the poultry smut. (during free ranging)
What I wish I could find easily locally:
Organic feed. Granted, it just became available locally, but they aren't quite consistent with keeping it in stock. I waited a long time for this and have been grinding our own mostly organic feed for a long time. We aren't switching over to their feed because we don't need to now, plus it's still too far to drive regularly when we know we have super fresh feed right here. But if your clientele is the sustainable crowd, then 9 out of 10, I'd guess, are not going to want to pollute their backyard/land with standard feed (contains unsustainable grains and legumes, right? Seeds that can't be saved from year to year)--especially if they're composting the contents of their coop and putting it on their gardens. But then, who knows? You yourself buy Agway feed. So I guess there may be many different definitions for the word sustainable. I would think that sustainable, beekeeping, folk would tend toward organic feed. Too bad you can't get a survey done before you open.
A wall mounted oyster shell and grit holder that is easy to fill and easy to remove from wall for cleaning.
Extra replacement gaskets for the 3 or 5 gallon red and white waterers.
Hen aprons aka chicken saddle (for when a hen starts pecking on her friend or over preening herself). Even without roosters, some hens will need these.
Covers for my feeders so that the hens won't roost on the top of the feeder! Stores should make these available if they sell feeders.
Wooden nest eggs. I'm using golf balls, but would prefer wood eggs. We've got a lot of golf balls out of commission.
Spiral bands! Assorted colors. I have been asking for these for a number of years and no one will carry them. Really annoying. If you have 3 chickens the same color, how can you tell them apart when checking on their health or learning their odd habits? Or figuring out which one lays the weird shaped/colored egg? Just adult hen sizes should be good enough. Just a couple bags.
Brinsea products. Any of them. They are quality items. I can order them, but it's just so much simpler to buy it locally, plus it helps your community. But the Brinsea products like the Eco-Glow brooder are unique and low on energy usage. The Brinsea incubators have provided me with great hatch rates. You may think that if there are no roosters in your locale, that there will not be a call for incubators. But many chicken folk have fertile eggs shipped to them so that they can have fancy and/or rare breeds inhabiting their backyards. Sure our local store carries brooder lamps and styrofoam-style incubators, but they could carry Brinsea incubators and brooders. It would be wonderful if they would just carry a simple tilt-your-own Brinsea incubator. They have shelf after shelf of turkey deep fat cookers right now, but supposedly no room for Brinsea incubators.
Rare chicks. I would love to be able to get some of the more interesting or rare breeds of chickens from my local feed store. They only order the regular chickens, though. I'd like to see them bring in Campines, Spitzhaubens, Faverolles, Marans, Dominiques, Exchequer Leghorns, Hamburgs of any type. So if you have Chick Days, maybe you can get some mixed specials for those who are wanting something out of the ordinary Barred Rock, Buff Orpington, Australorp style of chickens. (Nothing against those 3 very popular breeds, but ... please ... some more variety. Maybe those are the top 3 favorite chickens because that's what the hatcheries push on the local feed stores for chicks days.)
Egg Skelter. Coolest egg dispenser ever. It would take up space in your store, but I think they'd sell really well. Everybody likes them. British product, but US folk like them a lot!
A cloth doggy carrier for my Silkie to take with me on short shopping trips. This would depend on your clientele. But I'm semi-rural and I would like one just for fun, especially to take over to my friend's house who always brings her little doggy with her to my house. I'll bring my chicken and get a laugh (or kicked out).
Chicken diapers. A must if you want your chicken to come in the house or take trips with you. No, I don't think I'll every buy any of these, but plenty of folks do. If they were on the store shelf, I'd be tempted to buy some for emergencies or just the novelty of it.
Electronic egg scale (or one that would work for food also). If you have it, they will decide they need it and buy it. We bought an electronic scale for feeding our chickens and I used it to weight eggs back when I was checking egg sizes.
Cute chickening and gardening boots. Love the plaid or polka dots. Wellington style. Muck brand is carried all over. Servus brand - made in USA. Or royal boots like Diana's Hunters or Katherine's Le Chameau. Hunters are available in NYC, last I heard.
The Little Giant red, green, and purple chick feeds are great. Easy to clean. Easy to open without spilling. The galvanized feeds (brand name unknown) have a lot of sharp edges that I cut myself on so I'm not keen on those. Plus I spilled more feed with them.
Above poster mentioned Red Wriggler Composter. I would buy one of those. I ordered one online, but it never shipped because of ... something. I don't remember. If I saw one locally, I would snap it up.
For the grand opening, though, a really cool item might make folks sit up and take notice. I'd love to find a store that carried the regular stuff, but always had something really cool available, too. That would make me stop just to make sure I wasn't missing out on the really quirky or super practical items. I'm thinking Egg Skelter, but then maybe they're already old hat on Long Island.