The system that prevents the employees of the Sam Ash Music store in Midtown from buying stolen guitars begins before the case is even opened.

“First thing I say is, ‘What do you got there?’” said Ian Goldrich, a store manager.

If the seller’s answer is something like, “It’s a 1953 Fender Telecaster with an Esquire neck,” then it is probably legitimate.

But if the answer is “a guitar,” that’s something altogether different.

A new seller who entered Sam Ash on Sept. 28 had a plausible explanation for not knowing much about the contents of the guitar case he carried that day and, later, other cases. “His father moved to North Carolina and left him the guitars,” said Jeremy Kolmin, another manager. The store bought his Gibson that day, and $4,300 worth of other instruments, mostly guitars, in the weeks that followed.

“He’s a nice guy,” Mr. Kolmin recalled.

The staff at Sam Ash knows a lot about guitars. So when that same seller entered in late November with an electric guitar, Mr. Kolmin did a double take. He was sure he had seen it before.