Moved to Refund Request, but I'll bring up the possible bug with staff. There was a rollback around this time, and typically those are not refunded. But there also may have been a glitch with your town bank that's not part of the rollback.
Making sure I understand this right...
1) You owned a town called Diablo.
2) Your alt had a town named Grandeur.
3) Mylucky7 wanted to buy the name Grandeur, but not the town.
4) Three days later, you changed your alt's town's name from Grandeur to Crossbow and changed your town's name from Diablo to Grandeur (to reserve the name).
5) You then sold your alt's town to Jamiie_Londors.
6) Mylucky7 said he no longer wanted to buy the name Grandeur, so you changed your town's name back to Diablo. At this point it had 3 million Meebles in the bank (did you make a deposit around this time?)
7) You log back in later and the town name is Grandeur again, and the 3 million Meebles are gone.
If you made a deposit during the time of the rollback, that's probably what wiped the bank. If not, I saw couple of other towns that seemed to glitch out at the same time, and money that should NOT have been rolled back just disappeared. I'll check with rAwsondayc if he has screenshots of your /t deposit and /t withdraw commands (Edit: also, those of your co-mayors). The timing on those could help us figure out what happened to the money.
Do you happen to have any screenshots of the town balance during all this?