I was talking with my accountant, he said that even HRMC (UK tax) won't think that you are making a profit if you sell a designer item for a lot less than what they cost, of course receipts make things easier, but if the things are fairly consistent with the same size and you are not regularly selling, not even tax will think that selling Chanel shoes for 100 is "a profit", you can't even find them for that price in any charity shop, I just wanted them to go to a good home, as my taste has changed a bit and I don't wear navy anymore.
I have a folder with old pictures when I wore the items, just to show that they are really personal items. I have moved countries several times in the last 20 years due to work, it would be absurd to keep all the receipts or bank statements that go back decades, but I am only selling the odd bits and bobs, to have a clear out. Not sure how it is in the US currently, but here you can have a clear out as long as you let them know if you are over a certain amount and the reason for it.
I think they are trying to find the professional resellers who don't declare and are actually treating it like a job, so if your previous tax returns show you barely paid taxes on income, you might have a problem declaring how you manage to sell a few high priced designer items that are "personal items".