From my 4-5 trade-ins with Gazelle that seems to be their standard policy. Games that are opened have always been marked as "good" instead of "perfect" even if they are indeed perfect but they've never changed the value they give.
Does anyone else think it's weird they require the case/artwork but there isn't even an option to check for the manual? Dunno if someone else has posted this already but I sent in a DS game with no manual and they did not seem to care (got the full value).
I had a copy of Fallout 3 X360 (NEW) and a copy of Pure X360 (USED). However, when I sent them in, I marked them as "perfect". Both were open, not sealed, and accepted as "perfect". I would guess it depends on overall condition, but you do have more experience than me. Or, it could all be pretty YMMV.
Also, rather hilariously, my Fallout 3 case was missing a piece from the holder: it had broken off due to rough handling of the case (or poor/stressed plastic or both), the disc was rattling around in the case and looked a bit used. Using a microfiber cloth, I cleaned it up and it looked just like it really was: brand new. I have yet to find another good trade-in to Gazelle, but once I do, I'll let everyone know my experience with what they deem my games as.