I have seen some reproduction cards of very old early 1900s cards that are clearly marked that they are reproductions on the mailing side of the postcard. I don't list any of those postcards because there isn't a market for them and I feel that buyers would be upset if they received a reproduction postcard.

There are also reproductions like the Curt Teich linen style of postcards which I feel are perfectly fine to sell on eBay. These postcards always have numbers and letters on the picture side and they end with an "N". For example, the number may be 12B-7878N. The N means that it is a reproduction by Curt Teich. These cards are acceptable as long as you call out the number so that collectors know that it's a reprint card.