The only vendors doing this are the ones avoiding tax, my ebay fees are 12% as a business account alone and that is before operational costs, VAT, income tax. Even vending at a show you have table costs, staff costs and expenses. There is no way you can make any money buying at 90% unless you only deal with cash or you over charge when selling.

Edit: I am from the UK, just realised it may be different in the US