I guess what I would do at this stage is remove the buffer/spring and note the passing of the bolt/carrier for binding fore or aft.
In addition to using the charging handle - I would try to move the carrier to the rear by pushing on the bolt or carrier face to remove any torsional issues IF there is already binding using the handle.
Then, I would want the carrier to return forward based soley upon its weight and gravity.
Then, I would check the seating depth of the magazine.
Then I would look for signs of dragging of the bolt carrier within the receiver beyond normal wear.
Could be receiver, magazine or simply the main spring. Unfortuantely, you don't have a spare .50 to compare it to let alone more experience.
In any case, if you buy it, before letting any *gunsmith* work on it, I would perform the above checks, take as good a picture of the magazine seated through the ejection port as you can and make some notes before talking to Barrett about it.
Todd.