I wanna buy a engine for my olds aurora 4.0 from someone on ebay what do you think?

Buy local only. You can find a rebuilt or salvage engine. If you have trouble then you can go see who you bought if from and get it fixed. If you buy on Ebay etc If you have trouble you may never see them or your money again.

Unless you view the engine directly before buying it and make a few tests you are taking a significant risk.

You can minimise that risk if the seller has a lot of excellent feedback (don't rely on the score alone, read buyers comments as well), but even an excellent highly rated seller can be mistaken about what they're selling. Think of it this way: steam clean your cracked block, maybe paint it as well, and your pile of scrap will look nice enough to sell.

I haven't even bothered to look at the advert because all that matters is the item itself. So look at the feedback, ask the seller if the block has any known faults or cracks, ask how the seller knows it's fault-free and then view it directly. If you can't go and view it then accept you are still taking a gamble and factor that into what you're prepared to pay.

Until your mechanic has the block you absolutely cannot be sure whether it's a good usable item or a pile of scrap. Even if usable assume that it will need work.

That engine is exclusively unique to that car. It's based heavily off the Cadillac Northstar V8 engine and is almost completely identical to it save for the smaller displacement. As for where you're getting your replacement from, it's an auto salvage yard so they're probably going to sell you the entire engine with some of the attached accessories, or they might just sell you just the bare long block assembly (that's the block, heads, and all internals of the engine), and you will have to swap all the external components from the bad long block to the good one while replacing all the necessary gaskets. Either way you're getting a used engine so don't expect it to be perfectly sound. I mean some salvage yards test components to ensure functionality and even give warranties for a certain number of days guaranteeing functionality of the parts. It's the best they can do since they didn't manufacture the engine. They're just reselling it to others.

Why don't you just buy a used engine from your local junk yard and save a couple hundred bucks?

Try to find a local junk yard. Some junkyards will pull the engine and do some basic tests, and they may offer a short warranty on at least the engine, not the labor. Or since you are going through this, could someone locally rebuild your engine, just get a new block and use the other parts. So that way you have fresh piston rings, gaskets, clean value seats, etc.
Note that will only ship via freight carrier. It has to go to a commercial address, and they will need a forklift to get the engine off the truck. Usually the driver is not allowed to touch the freight.

It would be better to buy a used engine from a" local" salvage yard where they can test run it to make sure it is good or buy a remanufactured short block from local parts store. If your friend is a real mechanic, he should have sources. Good Luck

Looking at the advertisement, where does it say just the engine is for sale? You'd be buying the whole car, AS IS.
It doesn't even say if the engine RUNS.

I cannot do a thing for you.