You can buy unlimited gift cards directly from the merchant for face value, so that sets the price ceiling. Gift cards are strictly worse than cash, and individuals with gift cards who want to sell them value them less than face value, otherwise they’d keep/spend them. The only legitimate cases I can think of are if you are spending an eBay gift card to buy one you value more, or if you somehow get extra cash back or etc to justify buying from eBay over the gift card issuer.