After a car is repo can the finance company sell it back to the dealer you bought the car from or is it suppose to be auction off?

Normally it is not auctioned off but it can be. Normally they accept bids on it. The lender owns the vehicle and can sell it to anyone they wish, even the dealer you bought it from. You have no say in who they sell it to, and you are also going to be responsible for the difference in what it sells for and what you owe on it.