In most states, used mattresses are not allowed to be sold unless they have undergone deep cleaning and been certified as reuseable. Given that a used mattress can have bed bugs, urine stains and many other unseen problems, a used mattress is really worth very little.
A Good Deal
If you get a receipt of when the mattress was purchased and can use a black light to verify that the mattress is clean, purchasing a used mattress for up to one-third of the purchase price is a reasonable expectation.
Be wary of paying any money for a used mattress unless documentation supports its previous purchase date and its cleanliness. Refuse to pay more than one-third of the cost of the same mattress new.