If you buy $1000 of toys and then sell them on eBay for $200 later, then you won't owe tax because you had a loss of $800. However, if you were audited, you might have to prove that the toys originally cost you $1000. If this is the only unusual item on your return, it's unlikely that an auditor will bother with it.

However, if you went to, e.g., China and bought $200 of toys and came back to the US and sold them for $1000, then you will have to pay tax on the $800 of profit.