Two main reasons why the secondary market suffers:
1) Selling it privately (ie. Kijiji) there is a level of uncertainty from the buyer's side even with the proper documentation. Also, people expect a major deal to be had since it is private.
2) Selling to a random store, the buyer needs to get a good price on the piece, otherwise why not just purchase through regular suppliers?
Depending on the situation and the piece, best bet is to return to the store it was originally purchased from. If you can re-purpose the diamond or gemstone, you'll get most value. You could also look into an exchange or credit towards a new piece. A diamond never goes bad. We've had situations in the past where we'll give full credit of the original diamond towards a new piece.