Why on earth would a GS/1K/serious business traveler try to sell a measly $ 125 e-cert (or upgrade instrument? Whether on eBay or otherwise. It is an obvious violation of ToS, this being a no-no is noted on any ecert, and it does not matter that all legal details only might be found on page 17 in the fine print.
Moreover, everything is fully traceable - the ecert, the name of the individual it was issued to, an eBay account, any eBay transaction, etc. I don't know what a $ 125 ecert goes for on eBay. What I know is that I would not have the time to deal with eBay for a couple dozen bucks, maybe. I need this time to book my next flight.
I have broken plenty of ecerts (full or residual) over the last 20 years (UA and others). Such is life. Many of my RPUs/GPUs go unused or get used on SFO-LAX... And when I sponsor a family member or employee I call UA and specifically state the relationship so I will not run into any issues.
Penny-wise (not even that), pound foolish.