I didn't go last year, but went the three years before that so I feel like I know the tournament scene and Indianapolis well. A first day Gophers-Whoever matchup certainly won't sell out, as long as that whoever is not Indiana, or possibly Purdue/Illinois. Even then, you'll be able to scalp a ticket for face value at worst. After that, fans are just trying to get rid of tickets as their teams are eliminated and they want to go home. Again, the big exception is Indiana and to a lesser extent Purdue/Illinois, as fans will drive in for the day and snatch up tickets. Still not impossible to get tickets.
Even though Indy is about as central as it gets, the Big Ten is a spread out conference and I think it disincentivizes a lot of people from making the trip. It's also not the most fun to drive 10 hours to see your team blown out in an NBA arena. Best to go with the mindset of a trip with some basketball on the side. It's a fun atmosphere, with folks in attire from every team roaming downtown.