Overall, I would say that many I know would expect them for free. However, I don't offer eggs very often. As
@IamRainey indicated, they can be used in-kind like for the electrician. Funny, I do this with an electrician friend - he has a home improvement business, but he's been a commercial electrician, and has done electrical work at our house. He likes chickens and is appreciative of the free eggs. If I were to ask for cartons back, I'm pretty confident he would make the effort to return them. Others - not likely. But, if a parent has gone out of their way to pick up my kids, take them here or there, then I will offer up a dozen eggs if I have them.
In general, people like rewards, so in the case of selling eggs and wanting your cartons back, then a note inside indicating a discount on next dozen eggs for a returned carton in good (re-usable) condition would result in more cartons back. If you are giving away your eggs, but would like the carton back, then I would say adding a cute/catchy/pithy saying or quote inside the carton would also increase the carton returns or at least they might think about you when they have a store carton or two that they might think to give to you.
Only other thing that comes to mind is that each state has differing requirements for egg sales. Often limited to no real regulation selling from doorstep/own property, but increasing rules as you extend your reach farther, for example farmers market sales to selling to grocery or to a restaurant (with the highest rules/ requirements). I recall when reading through OH rules, at some point in the sales process, the cartons must be NEW, unused (but, I don't think this is for sales from your own doorstep). Re-using cartons from say a grocery store and there are rules around what needs to be covered up and what information needs to be provided (like number of eggs and "ungraded" statement). Anyway, all this to say that eggs do cost time and money, so you certainly can feel justified in asking for some cash!