NHL's Mike Bossy -- Legendary '50 In 50' Puck ... Nets $6,850 At Auction

The legendary Mike Bossy "50 In 50 "hockey puck -- the puck he used to score his 50th goal in 50 games back in 1981 -- now has a brand new owner who shelled out nearly $7k ... TMZ Sports has learned.

We broke the story ... the seller, Jake Jacobson, says he got the puck years ago from Bossy's daughter as a thank you gift for swim lessons.

Jacobson says he had offered to sell the puck back to the NY Islanders star for a fair price -- but Bossy didn't want to pay anything ... believing Jacobson should've handed it over for free.

Jacobson eventually took the puck to eBay -- and now we've learned it sold to the highest bidder at whopping $6,850!

So far, the identity of the buyer has not been revealed.

As for Jacobson, he tells us ... "I tried to return it [to Bossy] for too long. He could've reached me. He could've made an offer. Now, it's time for someone else to have the puck."