You were not paid cash if you sold on eBay, right? You were probably paid paypal and then transferred to your bank.

When you make the bank withdrawal they should give you a receipt. KEEP THAT! It is the key to showing the source of the cash.

Many people have $20,000 in the bank so you won't be asked about the original source unless you act suspicious. You just tell them you took it out of your bank account and show proof.

If you end up bringing more than $10k back just fill out the form accurately. If they ask you can show the source and go about your merry way.