I just bought a used 2010 Trek 7.7 FX for $900. The bike looks like new and rides very nice however, I'm very disappointed as the rear gears are slipping (shifting up and down) and I noticed a significant scratch on the rear derailleur "105". It seems like the bike fell down on that side or was dragged causing this damage. I'm taking the bike this weekend to our LBS to check it out. I was wondering, even if they are able to adjust the gears, would it be better to replace the rear derailleur to avoid any other issues in the future or do I just ride it for a while and see what happens? Your thoughts please.