Bazaar Question:

They mentioned something about not being able to sell certain 'rare' items or something.

So does that mean I can't sell gara bots/weapons or bots that are bought using rt or something?

Yes, they can't be sold/bought. Only UC stuffs and Hu-kun BD.

All UC items/parts/weapons can be traded in Bazaar. Everything that's RT, can't.

You can check in shop if parts and weapons are RT or not. All parts/weapons you currently own or have owned you can re-buy in the Shop. If the items are sold in the shop for RT, it means it are RT items and that you can't trade them.