The owner has already paid for that product regardless if he sells it or not. If you steal that product from the owner, you rob him of his ability to make whatever the markup would be on those product. Thats assuming he would sell it if it wasn't stolen. Technically he's only out what he paid, not what he expected to make.
But we aren't giving the markup so i'm assuming its like a physics question with no friction or air resistance given... you assume none.
If we aren't assuming 0, then he would lose the $100 worth of merchandise + markup
You mean minus the markup? Why would you lose the markup you added on TOP of what you paid as the owner? If he paid $95 for the merchandise and sold it for $100 (even tho the thief is using his og $100), the owner makes up $5 and is only out $95, not $105