Unfortunately, there is enough “worked fine when pulled” stories to make people question, and with good reason.
I’m going thru a 400 for a friend of a friend, it’s going in a turbo LS powered mustang (only way to make a ford fast!!). They guy he bought it from said it was rebuilt 10 years ago, and has been sitting since he sold the car 9 years ago.
This trans was obviously built by a YouTube certified tech. The guy knew just enough to get in trouble. He used wheel bearing grease to hold the thrust washers together and as assembly lube for the bearings. The grease was still packed in the bearings, that stuff doesn’t dissolve in ATF. The worst part was he got a couple snap rings in the wrong location. The snap ring didn’t fit properly in the groove, it jumped out of place, destroying the direct drum, center support, and case. The other parts were destroyed from lack of lube, I’m guessing the bearing grease restricted the lube flow. On the plus side, the frictions looked flawless. I’m guessing the trans didn’t last long enough to burn them up.
Paid 500 bucks for this turd, the only useful parts are the output shaft, pump, and oil pan.
To answer your question, if they guy will let you pull the oil pan for inspection, that would be a huge plus. Look for excessive crud in the pan, smell the fluid. Brown fluid doesn’t necessarily mean it’s junk, just means the fluid has been neglected. If it’s black, smells burnt, and crud in the pan, the price needs to go down. If he won’t let you pull the pan, that’s probably a sign he doesn’t want you to see inside.
I’d say 200-250 bucks tops for a core is a good price.
The place I buy my transmission parts won’t sell a remanufactured stock 350-400 converter without a core. The cores are difficult to find now. Who would have thought that would have ever happened?! I guess 40 years of people throwing away stock converters thinking “nobody will ever want this!” has come back to burn us. I personally have tossed many in the scrap pile because the scrap value was more than the core charge.