Just to warn about buying memory foam toppers on eBay.
The 3-inch memory foam topper that I bought from eBay is useless. It bottoms-out and is not up the job, even though the seller had an excellent feedback rating and had sold lots of them.
Because the eBay feedback ratings are for the sales transaction, rather than for the product, you can't tell how good or bad the product is. So you can get caught out with a dud product when a clear specification is not possible. For instance, there is no clear way to specify how well a foam topper works.