I sell eggs word of mouth, sign at the end of the driveway, and (next summer) at farmer's markets. I get $3.50/dozen, with .25 off if you return a carton.
We wash in unscented detergent with a dilute bleach rinse. ALWAYS use hot water to wash. Water colder than the egg will force bacteria into the eggs through the pores. Hotter water will draw bacteria out.
I live in Ohio. If you just sell from your premises (never deliver) there aren't many rules. If you deliver, as I do, or sell off the property, there are labeling laws and you would have to completely obscure any markings on re-used egg cartons. It's way easier to buy our own, put a sticker on, and then pay that .25 bounty on the returned cartons. Our customers like the re-use factor, too.
I sell a mix of colors, and work a bit to keep the cartons pretty. We try to get at least two blue and/or green eggs per carton, plus cream and various shades of brown. I've been known to do whole cartons of blue and green around Easter, but for those I charge $5!
The big point here is that if you plan to deliver eggs or sell off the property, there will be laws from your Department of Agriculture and possibly the Health Department.
Erika