$250-500 depending on condition
Download a manual from Browning or take it to a gunsmith
Depending on condition, from 250-450 USD.
You need to see a gunsmith.
Several, the auto5, B2000, B80 and others
It will lock after the last shell is emptied from the magazine
Browning never marketed a B80 specifically as a "Trap Model". In 1987 they came out with the B80 Plus model which could shoot all loads. Prior to that, each gun used a special recoil adapter. Invectors were also available. The B80 was dropped from the line in 1989. Basically any B80 with a 30" barrel and full choke could be considered a gun for trap, however the stocks were designed for hunting use.
Is it a Browning? If so,is it a BPS,B80,A500,Double Auto,Auto5,Gold,A-Bolt,O/U or SxS? What gauge? Your question is sort of like asking, "I have a truck. What is it worth?".