I bought a used 2002 VW Passat with 85,000 miles before I found this site. The Ford dealership where I bought the car from is long way from where I live but I was able to get an inspection done and I was able to do a pretty long test drive. During the test drive the check engine light came on and the car became sluggish. Everything else was great An inspection revealed that the car needed about $600 in repairs for the fuel injection system. I asked the salesman about lowering the asking price but he said they would fix the car.
Later, when I returned to look at the car again, they showed me a receipt for the fixed items. The cost was less than what was initially quoted but the car started up fine and the check engine light didn't come on.
It wasn't until I signed the papers and was on my way home that I noticed the engine became more and more sluggish. The car wouldn't even go over 65 mph after a while.
I took the car to a respected VW/Audi shop close to my home and they said not only were the wrong spark plugs used, but the induction system wasn't put back together correctly and some parts were sealed with silicone sealant that is not recommended by VW. The use of those parts caused additional problems with the fuel injection system and also caused an oil leak. Now, not only have to re-fix what the dealer fixed, I have to spend additional money to fix the additional problems.

I am thinking that I should've bargained for a lower price instead of letting the dealership fix things.
I was just afraid that the car might not have made it to my mechanic.