It depends on the venue. Check with each place and see how the money is collected and how the sales work.
With cafepress, spreadshirt, printfection, you don't need a seller's permit to sell via those sites. That's because those sites are actually the merchant that is selling to the customer. They are collecting sales tax on the end sale and reporting it to the state that they live in. You are just getting "commissions" from those sales.
Via etsy, I don't think you need one either, but it depends on if the customer is paying you directly and the laws of your particular state.
With PrintMojo, you don't need one if you don't live in California (where PrintMojo is). Since with PrintMojo, you are actually purchasing custom printed merchandise wholesale, if you are in CA, in order to avoid sales tax on your inventory purchases, you would need a CA seller's permit. If you are outside CA, then you wouldn't need a seller's permit.
If you're selling via eBay, it might vary based on your state's laws. For example, if you are collecting sales taxes on your ebay sales, then you need to have a CA seller's permit from the state board of equalization.
If you're doing it as a hobby, you *may* not need one. But most people here are selling as a t-shirt business, so if you are in CA, you would need to be reporting your sales to the state. Many other states are similar.
So I guess the answer is: "it depends"
