Buying an engine from ebay... GOOD OR BAD?

I’ve have bought 3 engines off eBay. Although they should have been treated as cores, I paid up for them at the time prices as such. Was it taking a chance? Yep, but I’m good with that.

1957 GMC 302 straight 6 cylinder. Complete engine. I have about $500 into it including driving to Minneapolis from Milwaukee to get it, with renting a pickup truck. Farmer pulled it from a truck to put in one of his, told me it ran when pulled. Told me it had less than 50k miles on it. Right now a block alone is about a grand and they are getting hard to find.

1972 340. I have about $500 into it including shipping from Seattle. Turned out to be forged crank.03 under from factory. 1.88 J heads , no intake, carb,distributor or water pump. Someone rebuilt it using stock bearings and it was never ran. Guy said he bought it, never used it or tore into it. Stated he had no idea what was inside as he bought it as a rebuilder. Rare engine in my opinion and a piece of history.

Pontiac overhead cam 250 straight six, year unknown. Complete engine. 4 barrel motor including the manifolds and carb. Hard to find carb. I have $400 into it including driving to Indianapolis to get it including renting a pickup truck. This engine was spotless and was on an engine stand that he threw in.

Even though I have not gone through these engines, I’m good with it all even though it’s rolling the dice. After having these for 15-18 years, bet y’all I could double my money if I was to sell them.

My experience has been that high performance engines/parts never go down in value, only up.
Both of the sraight sixes have cult followings.