I've always wondered what is the difference between selling items on eBay vs. selling items at a garage sale. I've never heard a tax rule that you had to claim garage sale income on your taxes unless your selling something that has appreciated vs. depreciated (like 99% of most items). I assume one would have keep track of the original purchase price of an item being sold, which would be cumbersome.
On the other end, if you buy something on the internet typically you should have paid a sales tax. On the Ohio income tax form, they ask for the amount of your purchases on the internet that did not collect sales tax and then you calculate the appropriate sales tax and add it to the tax due (or subtract it from your refund). Again, I've never seen sales tax collected at a garage sale.