So, here's my next question: do we owe taxes on garage sale proceeds?
You most likely do, if you are looking for the "legal" answer. You prob won't be asked to pay it if you have an occasional garage sale, becasue the IRS usually assumes you are reselling your own items, and for less than you paid. Can they insist on income taxes? Yes. Have they gotten income taxes out of someone holding a garage sale? Yes.
I think it (should) depend on whether you are using ebay to sell your junk or using ebay as a business.
That would be nice..but it's not the way the government thinks. If you are selling occasional junk, will they come after you? See above. Lots of people think it should depend on if it's a business..but in this country, it just isn't that way...and think about it...just on the ebay thread here..how many are buying just to resell? Lots..after all, you just have so much in your closet. Most all of the people posting on the eBay thread have a business, if they want to call it that or not. Believe me, if you fill out a schedule C properly, the average Joe, isn't going to owe a lot of taxes if all they are selling are garage sale type items on eBay, by the time you take off your deductibles and expenses. and fees...but you have to fill out that schedule C to get those deductions.
shouldnt need to pay tax on it (didn't I pay the tax when I purchase it to begin with?)
You prob paid sales tax (not income tax) when you bought it..but in this fine country the same item can be taxed over and over, and in most states, when you sell something within your state, you are suppose to collect and then pay to your state, sales tax. If you sell something on eBay, and want to claim the costs so that you can figure out your profits, you need to fill out a schedule C. When you fill out a schedule C, you can put down what your receipt shows you paid for the item, and since you paid tax, on the Schedule C, you'll get to take that off your profit. So you won't pay income taxes on it. If you don't fill out a schedule C, you have no way of taking off your costs.
Remember..the OP was asking what the correct way was..if she would owe taxes. The "legal" answer is yes.
This answer is not meant to take the place of your own local tax preparer or accountant.