Hi Ben,
I bought my Sealy latex mattress off Craig's list about 6 months ago; I happened to see the very one I wanted to purchase new but didn't have the bucks (2 boys in college right now). It had been used a year and too hard for the young couple who bought it. It was in great condition and cost 1/5 of the retail price. Some people can't handle the thought of a used mattress, for a number of reasons. Because it was clean and in perfect condition, I considered mine a heavenly gift!
I got it home and it indeed was too hard, even through the one I was replacing was even harder. Once I added a couple toppers it's been fine. Most folks are selling mattresses because they are not supportive enough due to the cheap PU foam layers that have created indentions which end up hurting your back by morning. I'd guess 90% or more used, surplus or outlet mattresses fall in this category. You must do the hard work of educating yourself and knowing exactly what you need and who makes it and what a good deal is, once you know you don't mind buying used.
Knowing about Foam by Mail is a huge blessing. Their products can make a hard mattress work for you, but not the other way around. Best of luck.