According to Dan Pompei, "other teams believe guard Davin Joseph could also be had in a trade (h/t The Pewter Plank). And I'm sure those teams also believe Santa Claus isn't real (sorry kids), Lance Armstrong is a delusional cheater, and Parks and Rec is a hilarious show.
Yes, Davin Joseph is available for trade. Any player slated to make $6 million, coming off one of the worst seasons of ANY NFL starter at his position and over 30 can be had in trade, especially when there's a lot of speculation about him being outright cut. Probably for as little as, I don't know, some backup lineman who probably won't make it out of training camp. But preferably draft picks. The Bucs could really use some extra draft picks.
That said, Joseph is not entirely without value to the Buccaneers. There's a chance he'll bounce back and that his poor season was solely a temporary result of still recovering from patellar tendon surgery. There's also a very good chance that Joseph is nearing the end of his career, and will never recover from what is a very serious knee injury. But Joseph also has tremendous value in the locker room, and he at least knows his assignments and can execute plays as they're meant to be executed.
Losing Joseph would mean surrendering some security and depth along the offensive line, and the Bucs would have to sign or draft a starting guard. Probably sign one, because being forced to select a guard with your second-round pick because you have no other options is not ideal. But signing a new starting guard is likely necessary anyway, and getting a few picks in return for Joseph would be a tremendous return.