It is NOT safe to buy or sell UA upgrades on eBAY.
Ebay will release the BUYER's and SELLER's info to the airlines after the transaction.
If your UA account & eBay acct share the same info (for example, name, address, email address,
or phone number), the airlines can then link the account to prove that you are buying or selling upgrades.
I have a friend who got his UA account suspended after selling his GPU upgrades on eBAY.
He found out that eBay disclosed his info to UA.