I've seen some people use soda straws to keep the chains from getting tangled. You just cut a lenght of a clean straw and thread the chain through. Then clasp your necklace as usual.
I typically use an index card with two V cuts in the top. Thread the chain over the notches so the pendant, design, etc. shows on the front with the rest of the necklace on the back. Then I slip it into a 4 x 6 craft ziplock baggie. This lets me write info on the back of the index card such as types of stone, chain, etc and the price
Good luck with your selling!