Longevity? I've been at this a year this time. When I had birds before I sold goofy little Bantams for $1 each on chicks and $10 a pair for older birds. The only "trick" I have now is location. Our county fair has gotten to where people are surprised they still have pigs there, even if it is only 8 of them. Chickens that used to fill a whole building barely make 2 lines of tables, and that's mixing chickens, ducks, turkeys, geese, and the pigeons. They actually sold the fairgrounds last year to move them to a more rural part of the county in an attempt to revive the typical county fair scene.
If you decide one day you want backyard chickens, you can buy 25 up the hill at the hatchery, or you can drive an hour or so in any direction but local and close to home. We're so developed, anything farm related is far away. We have 4 Tractor Supply stores, not one of them closer than 20 miles away. Most of my sales are going out to the country, to people who want to start a flock to supply the urbanites. Some go local, to small flock keepers.
I advertise locally through word of mouth, our local poultry group, and CL. Started a waiting list last fall for the ducks, chipping away at that. Eggs are $20 a dozen, babies are $5 or $10 each depending on what I quoted previously. $10 each now that we're using the improved feed. It's $18-$22 a bag! If I have babies long enough to know gender, they're $30 a pair until they're fully feathered and I can see markings, if they'll be pet, breeder, or show quality. Pet quality is still $30 a pair, and on up from there. The ever increasing prices means I sell them early, which means I feed them less.
We don't have the space for quantity, so we go the quality route. I REALLY want Cream Legbar chickens... auto sexing AND blue eggs, and I've never, ever seen them locally. But I can't have a rooster yet, so just just ducks and laying hens now. I do sometimes buy rare chicken hatching eggs and sell the excess. Usually I do a little better than breaking even on the purchase of the eggs, and I end up with fancy layers. If I decide to sell them, I'll get more than $5.
There's a lady here who buys sexed chicks buy the hundreds for the discount, then sells them at $3-$5 each depending on breed/age. Another lady has a brooding service, if you want a duck from her it's $16, chickens are $8. She does a bulk order of what everyone wants, raises them till they're off heat, then you pick up your birds. She buys straight run to save money, processes the roosters for her own table. Others create sex link babies, selling in quantity pre-sorted babies, boys for the table. Others grow them out, selling point-of-lay pullets for $20 each. Selling eggs for eating, and growing out market chickens through pre-orders, only buying what you've already sold, usually at $2 a pound or so.
There's another lady who lives on 7 acres in one of the worse neighborhoods of the city. Not sure how she found that acreage, but she makes bank off of Turkeys, $7 a pound! Can't raise enough of them, expanding every year. Usually you need to go clear to Indiana or the next county over for a Turkey like that.