Here is what I expect to happen, you can believe it or not, but think about it.
Assuming that they have your SSN, eBay will be creating the 1099‘s and sending them to you and the IRS, you don’t create your own 1099. If they don’t have your SSN I don’t think that they can complete a 1099 which means that they probably aren’t reporting to the IRS and all this is just noise; maybe they’ll send you an annual summary, who knows. Lets assume that they have your SSN somehow.
eBay has no idea what your basis (cost) was for your bike. They’ll send a 1099 for the gross amount of your sale since that’s the only info they have.
The IRS computer will attempt to match that 1099 to your tax return, if it doesn’t show up you’ll get a letter, bill, audit notice, or something else mildly unpleasant.
Most likely you’ll put the amount from the 1099 on your return as gross income, subtract your basis and sales cost, and report the greater of the net gain or $0 as taxable income.
So, your brother is right about only paying tax on profit, but the 1099 will be for gross sales and he’ll have to report the gross sales and then be able to support his costs in order to prevent the IRS from assuming that it’s all taxable income.