The
UT ticket officeis still selling tickets to the opener in Charlotte with West Virginia and two different three game packs. The three game deals are pretty good because they cost about the same as what one ticket to a big game might cost and you can sell the tickets you don't use.
Vivid Seats is the official resale site for UT sports.
StubHub is the other major resale site. I have bought and sold tickets on both sites many times and I have always had good experiences.
As for when to buy, it depends on several factors. The games against smaller opponents will always be a pretty good deal - close to or below face value. SEC opponents will cost more and the big games, Florida, Alabama, etc., can cost a lot. How well the team is doing affects the prices. I couldn't go to the Vandy game last year and sold my tickets online for $10 each on the day of the game and felt like I was lucky to get that.
Which brings up another factor - timing. You can usually get a pretty good deal buying early (Florida, Alabama, etc. excluded), but if the Vols are doing well, prices will rise closer to the week of the game. However, if you wait until just before the game, a lot of people will drop their prices just to get rid of the tickets.
Hope that helps.