NBA's Richard Jefferson -- Buys Hat Off Dude's Head ... For $700!

You ever see a fashion item that you just NEEDED TO HAVE???

NBA star Richard Jefferson did -- a Vans cheetah-print snapback baseball hat ... and he paid $700 to buy it off some random dude's head!!!

RJ was at Hi Fi Kitchen & Cocktails in Scottsdale, AZ Saturday night -- when a dude named Andrew walked in with the cheetah cap.

Andrew tells TMZ Sports ... the Dallas Mavs star beelined it over his way with cash in hand ... and started making offers for the dome piece. Andrew declined at first ... but ultimately gave in for a handful of $100 bills -- 7 in total.

So, why was Jefferson so obsessed with the hat? Andrew says RJ actually purchased the lid for teammate Chandler Parsons ... who was also eyeing the cap that night.

The best part of the story -- Andrew says he originally bought the hat for $25.

Not a bad return on investment.