What about collectables? Do they count towards the limit, or can they be sold before you move like sculptures and stuff?
They would count. You would be buying something so they would have to count.
Can you buy and sell profitable items from the consignment store without placing them?
If you buy anything from the consignment store it still counts against your two per Sim limit.
And as for real estate, can we develop it as much as we want, or is buy/build mode restricted for real estate?
Buy/Build mode is restricted across the board. I'll clarify that.
EDIT: Okay, check out the first sentence of rule 7. I just edited it. I clarify that you cannot move into empty homes that you would build on empty lots. All moves need to be to preexisting homes. I can't imagine anyone doing that anyway since Build mode is indeed banned across the board.
Do we have to buy the house furnished or unfurnished?
Correctly answered above.