I am curious now, how do you know you booked the last two seats? If there are seats still open for sale, then the aircraft may not be full yet. In fact, I know it has plenty of seats left as it is only booked to 87 of 112 seats in the back.
If you took the last two seats, meaning you took the last two preassigned seats, then that would tell me that the flight is very likely to be full, and possibly oversold, but understand that we don't allow all of the seats to be preassigned. The last few rows are saved for the airport to check in groups, large families, etc. The Economy Plus section can only be preassigned if you have a qualifying fare or are a Premier member.
The last flight of the day, can be one of the most open flights, due to people trying to standby for an earlier flight all day, or if can be the most full flight, due to irregular operations/weather/previous cancellations, etc. So its hard to say if this flight will truly be a flight you could be bumped off of. Even then, we VERY often get plenty of volunteers that want to be bumped for the compensation. I would, its a sweet deal if I have flexible travel plans!
Okay, let me answer your question. For those volunteers, they will be offered a free round trip ticket that is good within the lower 48 states. Often times, depending on the situation, you may be rebooked on another carrier or provided a hotel and rebooked the following day. Let me know if you have any other questions.
Thanks for choosing United!