Yep, black goop coming off the chain is not good. And as for trusting sellers it can be a hit and miss sort of deal.
First of all I'd clean the whole shebang including the derrailleur pulleys. This would in clude the bearings and pins of the pulleys as well. Then oil it and look up info on the Park Tools and Sheldon's website and read up on how to adjust the rear derraileurs. It's knowledge you'll want in any event. While you're at it measure the chain. When you line up a 12 inch ruller at a pin then the pin at the other end should be within less than 1/16 over 12 inches. If it's much over 1/16 inch past 12 inches then the chain is shot and you need a new one. If it's up around 1/8 inch over then it's dead meat on a stick without any doubt at all.
If the chain is dead meat big time I'd suspect that the cassette is worn as well, at least on the more used sprockets. However start with a new chain and if it's not running far more quietly then look at a new cassette.
Normally the chain is very quiet and the usual freehub pawl noise is louder. If the drive train is louder than the freewheeling noise then you've got troubles.