Technically, you are supposed to pay taxes from the first sale.

However, if you are selling items at a loss, like I am right now (comic book collection that has lost it's value), you do not need to pay taxes, as it is like having a garage sale, you are taking a loss. This could be written off on my taxes to my gain, as they were bought as an investment.

In my opinion, but not the opinion of the fed, there should be a testing time, where you test market your items to see if they are profitable, to which you can claim it was a hobby. Then after so many months or so many $$, you need to start claiming the sales. If it is an item you purchased from a wholesaler/manufacturer for resell and not just garage sale stuff/fw stuff, you may want to start claiming earlier on and pay estimated taxes.

I think $1000 IMHO, is a good place to start.