The key to success (by that I mean, making a living doing it) and failure of any business, including a home based, internet business is selling what people need and want AND are already buying!
You can do research on sites such as Amazon and Ebay and see what are the hot or best sellers.
Then from there, do some keyword research using the free Google Adwords Keyword research tool to see 2 important things:
1) How many people are looking for that item to buy online?
2) How much competition does it have?
See, once you have found a profitable market, niche and product(s), you want to make sure that it is easily "reachable" by you with your skills, time and money.
There are still plenty of lower competition products that can make you money online.
However, having done a little research myself, I don't think mattresses would be where I would start.
I hope that helps.
Let me know if I can help you further.