Has anyone sold a used mattress upon receiving a new one? I purchased mine in May/June and found it much too firm. I am stuck and posted a question in here for advice about soft mattresses. As long as it is legal, I'd like to sell the used mattress on Craigslist or elsewhere. It cost $1200 new, is in a non-smoking, no pet home. Has no stains, has been covered etc. Nevertheless, it is used, but in otherwise fine shape.

Does anyone have advice on selling? Any lessons learned? Just how much/little would you pay for a good used mattress?