The Bills have retired only one number in the history of the Franchise, Jim Kelly's #12. Therefore, #78 is technically available, although I'm not sure the team would be willing to give it to anybody, even Super mario.
Just because #78 (and #34) are not retired, I'm sure the Bills organization as a whole discourages players from selecting those numbers, perhaps even refusing to give them out.
Because the uniform design changed, I don't believe the players were responsible for buying up the stock of their old numbers. I'm sure the league is not a huge fan of old style jerseys that aren't official throwbacks even being on the primary market.