I'm not really sure...I just got a tax ID, also knows as ein..I don't don't think you need both, a sellers permit,tax id, or resellers license should work. At least that's from what I understand,all you need is one to legallly sell stuff and report everything you earn to IRS doing tax time.
Some websites might require you have one or two of those things though, but me, I just got the tax id cause it's free at the irs.gov website, they assign you one right away too. I'm not sure if the permit or resellers license is free or not...