I also bought a turntable already 'set up' by a seller. It sounded like crap and I blamed the records for a long time. Then I upgraded the cartridge and discovered the old one (set up) had every single setting wrong.

If I was you I would assume the cartridge is not set up right. I would find the best instructions I could find on line, buy a stylus gauge, and most importantly (for me it was), insure the azimuth is set right.