Bought used bike, Shimano derailleur does not work - how to repair or replace? So I'm new to biking and I purchased a really good bike at a reasonable price. It's a 7 speed bike and as you can see the derailleur is messed up. I'm not sure how to find one that will work. I found another Shimano Deore model: RD-M53P SGS. Can anyone tell me if this will work? If not do you know one that will? Thanks a lot.
The derailleur in the picture has been scratched and the cable is connected wrong, but it should work if you install the cable correctly.

Searching for "RD-M53P SGS" does not find anything except this question. However, all Shimano rear derailleurs specified for 9 speeds and below have the same actuation ratio, so any of these should work. The derailleur in the photo looks like 9-speed model and it is installed in 7-speed bike.