Theoretically they can be sold as soon as they're weaned, which is at about three weeks, it's just a bit upsetting and stressful for the poor mites to be weaned and taken away from their mother and siblings all in one go, so it's one of those really bad ideas
I think the ones in pet stores tend to be somewhere between four and six weeks, and most breeders keep theirs for a few weeks longer -- one breeder's website said they sometimes kept the babies up to ten weeks if they felt it was beneficial. (I don't remember who the breeder was, aside from they were a dwarf breeder; however, there's not that must difference aside from the potential caging issue.)
Price-wise, look at how much they are in pet shops and do it a bit lower, would be my advice. (I agree giving them away free is attracting the
very bad owners.)