Is my reason flawed in thinking this way or is this pretty much doable:
I'm not sure if I want to play ESO but with GCU and the $10 GC, would I be safe to assume that I'd be able to get $40+ around the next 30% or better trade in and break even? If so, be willing to go tomorrow after work and do so and be at the worst with a title that I could use toward reaching Elite.
Not really flawed, but that's not 100% how it works. Typically the first two to three weeks (or more) $60 MSRP titles are $40 base trade in, making them $44 w/ GCU. When they did the bonus 30% and I traded in Witcher the 30% actually knocked it down a buck to $43 because they lowered the TiV for the 30%. If you notice, Witcher's base actually increased from $30 last week during the 30% to $35 this week without (it went from $40 to $30 during the 30%). So, typically even if you hate it you will actually make a few bucks when you factor in RZ stuff.
Alternately, if you want to be extra safe you could preorder it in-store for $5, wait to pick it up until the TiV is shown on BBY's website and figure out if you want to return the preorder or buy the game.
I typically will buy almost everything that comes out with a $10 credit, play it, trade it in, and roll it over to the next one. Most stuff I trade in, a few I won't.