I was wondering what this means
It could mean a few different things.
It might just be a build up of dirt on the heads and guides that the owner never cleaned off properly and let it get to a stage where Q-tips and alcohol won't be strong enough of a solvent to remove that grime. Stuff like Methanol or paint thinner might be needed to truly clean it up?!
It might also mean that the heads are worn out.
It might be a question of the owner using old worn out tape with edge track damage to the oxide layer of the tape.
It might mean that the transport, (heads and guides), needs to be demagnetized.
It might be a combination of all of these things or none of them too.
Your best bet is to get an estimate from a qualified service facility to really get to the heart of the problem.
Will the owner allow you to do get it checked out by your own mechanic, as they do in the car world?
How much is he offering the unit to you for?
Cheers!
