It's hard to say, something could have been tweaked way hard. Then again, where did you try it at, in the pawn shop.... was it on a metal shelf, were there shelfs around it, was there flourescent lighting? If so how much, and how close, was it the newer solid state lighting or an older set with big ass ballasts humming away? Stuff like that can affect a machine.
Then again, it's a gold machine, not an iron machine, maybe it didn't see the iron, if you are nugget hunting you don't want iron you want gold... if it's ignoring iron that might be a good thing, Id try the test with a gold ring, see how it behaves... and test it with one of the other machines next to it.... that way when you say it only got 8 inches where another can get 10 and a half, they were both tested under the same conditions and you know one wasn't biased by something external.
Worse case scenario... it needs fixing up... How much does it cost new? How much would it probably run to send it back to the factory and have if fixed? Is it worth it to you. On that though, pawn shops have a hell of a markup, you bring your machine in and your wrench and you show the guy hey, this thing is not all it's supposed to be, I am interested but if i did get it, id have to send it off for repairs... that price is kind of steep for a sick machine, you can do better than that.... see if you can get him to knock off another hundred or so, I bet he will. Then it will be worth your while.
I picked up my first metal detector, a whites spectrum for $150 bucks that way.
Aaron