2002 Ford Mustang Maintenance

My 2002 Ford Mustang was making a ticking noise, so I took it to Pep Boys for a diagnostics test. Also, my car smelled like gas. They told me that the noise was because of the clutch and that I needed a new one (and a fuel pump). So I took the car in a couple weeks later to get I fixed. I paid $1,500. The only reason I fixed it is because I'm trying to sell it. The next day a buyer came to look at it. We took it for a test drive, and I noticed that the sound was still there (even louder). I took it back to Pep Boys and they're telling me I need to pay $2000 to fix it. Shouldn't they have told me this problem when I had gotten the diagnostics test? Since they told me the noise was because of the clutch, is there anything I can do?