Hi needaewmattress,
Whether it would be “worth” selling would depend on any other options you have available (such as using it in a guest bedroom or as a spare bed or giving it away to someone that you would feel good giving it to) and on how much you could sell it for (which would depend on how much someone was willing to pay for it) but a good starting point for a used mattress or topper that is in good and “nearly new” condition would probably be somewhere around 50% of either what you paid for it or 50% of another mattress or topper that was very similar if a comparable quality mattress or topper is currently available at less than what you paid for it.
From the point of view of a purchaser this would be a relatively high risk purchase (especially if they aren’t local and can’t test it before they purchase it) because of the lack of warranty and because there would be no recourse if the mattress/topper doesn’t turn out to be suitable for them to sleep on.
There is also more about buying a used mattress in post #2 here.
Phoenix