As far as I know, The Dump does not sell used merchandise.
However, the mattresses they have are memory foam mattresses.
It's less a question of 'where to buy', as it is 'what to buy'.
With the mattress industry having gone bad...steadily reducing in quality over the last 15 years, it's all about avoiding memory foam.
Memory foam is the worst upholstery material ever invented.
Not only does it perform poorly...sleeping hot and needing frequent replacing...it off gasses the toxic chemicals from which it's made.
Not only is it a loust product, it's a health hazard...adding insult to injury.
Stay out of The Dump and away from any mattress made with memory foam.