Except that your broker knows how much you paid for the stock that you sold. So, the 1099 from your broker lists your capital gains and dividends.
A 1099k from e-bay or paypal will only list the amount they paid you. I'm guessing that the taxpayer will have to fill out Form 8949.
The Form 8949, when filled out will calculate the capital gain or loss and whether the gain or loss is short term or long term.
The big unknown is what the IRS will accept to document the price you originally paid for the item you sold, if you don't have the original receipt.
For example, will the IRS accept a line item on an American Express Statement?
I will bet that most people that don't have receipts will simply pay tax on the entire amount paid by e-bay.
There's going to be a lot of PO'd people.
Not if you use more than one broker….