TMZ Sports has learned -- someone completely unaffiliated with 20-year-old McCain has been reaching out to people on the platform recently ... and after weeks of talking and building up trust, the user will eventually offer gifted items from SKIMS and other companies at a discounted price.

We're told some users have fallen for it -- after all, Kim Kardashian's brand recently partnered with the NCAA for March Madness ... and McCain was all over the campaign.

SKIMS also has a relationship with the NBA ... McCain's next employer.

Once a deal is agreed upon, the hopeful buyers are told to forward money -- sometimes hundreds of dollars -- to a CashApp account that doesn't feature McCain's image or name ... which naturally sparks concern, but the imposter assures them that's only to prevent ladies from sending him money to get his attention.

In other words ... the person behind the swindle gives an explanation to any red flags that might be raised throughout the transaction -- and as soon as they get their dough, they either ghost or block the hopeful buyer and move on.

We're told people in the former Duke star's circle are aware of the account ... and are in communication with Snap to get it deleted.

McCain -- who already had money from NIL deals in college -- is a future NBA star on the verge of making millions at the professional level ... and has a massive fanbase on social media.