EXCLUSIVESam Cassell has something in common with John Mayer -- they both bought expensive watches from the same guy ... a guy accused of selling FAKES ... and now the former NBA star tells TMZ Sports he's bringing in experts to see if he got ripped off.
We broke the story ... Mayer is suing Charlie Sheen's BFF Robert Maron -- claiming he bought $5 million worth of Rolex watches from the guy ... some of which turned out NOT to be 100% authentic.
Cassell tells TMZ Sports he also bought a watch from Maron -- "It's an A. Lange & Sohne. It cost me a nice piece of change."
Sam wouldn't say how much he spent on the watch -- but A. Lange & Sohne pieces can run upwards of $150k.
Cassell says he was introduced to Maron by his former Clippers coach Mike Dunleavy -- and bought the watch from him because he offered the lowest price in town.
"What am I thinking ... this guys gonna rip me off? No. Mike got watches from him also. But at the same time, Mike's a basketball coach, not a jeweler."
Sam says he's now sending the watch to an expert to analyze the timepiece ... and says, "If it comes back fake, I'm going to the Dept. of Justice!"
For his part, Maron insists he did nothing wrong -- and claims he was always honest about the watches he sold.