So last night i took the batteries out of my e-bike to charge, and i put the bike under the stairwell in my building. This morning i woke up and the bike was gone.

Does anybody have any advice about what to do or where to buy just the frame of a bike or where to sell batteries. I'd appreciate any information on the subject.

A quick thought, it may not be stolen.
If there was a fire inspection, the management would have moved it. They wouldn't necessarily tell you. Try looking in the nearest official bike parking spot.

Ask the management about this. The gate guard would not know if he was not on duty.

Failing that.
Look for a place that sells second hand ebikes. I guess a number are stolen. They may be willing to buy your battery.

The only other thing I can suggest is that you buy the same bike, and then you will have a spare battery.

I would also complain to the management about the security guards who must have seen someone push a bike without battery and not challenge them. We got compensation for a stolen laptop in one place. We negotiated, they would pay half, but we had to provide receipts.

Also inform police. If your bike cannot be peddled, it is possible that the bike has been pushed into a corner somewhere else in your development, until it can be picked up by van. A search may turn it up, but you have to be quick.