It depends on how high the pick up line is on the reservoir--if the pickup line is about the same level as the oil--that could be your problem.
I'm not familiar with the Bobcat line enough to know whether the pump is direct drive or belt driven. If belt driven, check the belt. Did you ask if the unit has been serviced? If everything else is good, I'd start by changing out the hydro filter--it may be loaded.
It's just my opinion(which often is not worth much) that purchasing a Bobcat from a guy who doesn't know much about it OR without giving it a test BEFORE closing the deal is risky. Such a buy can turn into a money pit if you're not knowledgeable.