Well, let's see, first of all, by using a relative's
bank account for the transactions, you are exposing
your relative for potential IRS, state and sales taxes.
The good news is, as of this date, eBay is not reporting
your sales to IRS or anyone. So no one knows how much anyone
is selling eBay, except you. And even you don't, if you're
not doing accounting or downloading your reports in time.
(Before eBay removes the reports for old periods.)
The bad news? IRS is very much interested in eBay's transactions.
Sooner or later, their attornies will find justification for IRS
to audit eBay and also to trace the income from the Power Sellers'
accounts to see if the seller is reporting the income.
So, be sure that your relative is reporting ALL the income,
for income tax purposes. It won't cost your relative anything
in taxes because all the profits are being paid to you. So the
net Schedule C should be -0-.
Back to your original question about sales taxes.
For the present, the only state you need to collect sales taxes
for is the state in which your relative lives. In other words,
you are showing a home base for eBay of the state where the
bank account is. That state is your company's US nexus.
Incidentally, Vertex is a good site to explore about sales tax issues
At this time, neither the IRS nor the state know about
your transactions. But since they will... protect your relative.
Remember, when it comes to sales tax, you don't pay it.
You collect it from shoppers and pass it on.
Your relative will need to get a sellers permit in his state
and to file the annual or quarterly tax returns.
I hope this helps.