what should i do is it worth buying how much would be the cheapest to fix it cause the price of the car is 2500 is it worth it WHAT DOES IT MEAN

It means you need to replace the engine, because it's seized.
I'm going to vote to pass on it, since it requires a lot of mechanical skill.

It means the car needs a new engine, and you're going to spend more on that than the car itself. Early 528s were/are junk. Avoid it like the plague.

Usually when I scan the classified for a car, the words "Engine Locked Up" and BMW are two things I don't want to see side by side. Actually, I don't want any car I buy come with an engine I know I'm going to have to replace.

Lets put this way: The car will cost about 2,500.00. Not bad. The problem is, the engine ( even if you buy a re manufactured or used ) could cost as high $5,000.00. Even that will depend on how long the car has sat that way. If you figure all the sensors and other electrical components on the motor have rotted, then it will cost a lot more. Not to even mention how many parts are dealership only.

I learned this about 8 years ago over a SAAB. The guy wanted $350.00 for it. But it would have cost about $3k to replace the rack and pinion, replace both blown power window motors, and replace some of the weather stripping along the door windows.

Best thing to do, is not buy someones blown up garbage. There is no telling how that motor locked up. If the owner drove it into the ground, then chances are, the engine won't be the only worry. Use good judgement and buy a car that will actually get you from point A to B.
Hope this helps.