Thanks for that idea @Y2Keglide

Thanks @Gorn for the input too. I was able to have a mechanic inspect it and complete the tasks I mentioned above. Oddly enough, he didn't find the heating core to be an issue, but I discovered upon driving. I wasn't able to have him further inspect due to time constraints. I have been able to drive it for short distances (say 10 minutes), but the temperature issues become a concern at a certain point. With that being said, I am being as honest as possible with potential buyers about the condition. Can't say for certain on the paint, as I don't have the entire history of the vehicle available to me. 10k seems like a fair asking point from what I can tell...

 

Just remember you can always go down price. If you think 10 is fair start at $12,900 and say negotiation in person only else you will get a ton of "will you take this" emails. Just expect people to be a holes. Selling a classic can be a pain because you get all the guys that wish they could buy it wanting to talk to you and they want to take a spin around the block. I do not mind talking cars but that is what car shows are for.

 

Is it no longer leaking inside the car? I find it hard to believe any mechanic worth his salt would not find that problematic and a heater core issue.
If it's not loosing coolant but it's overheating it might be as simple as a stuck thermostat,pretty simple to replace or you could just remove it but as long as it's apart a new one isn't very expensive.
It's only a couple of bolts and a gasket,about a 10 minute job.

 

If the heater core acts up you could just get two of these. As long as the hose is not too brittle.
https://www.harborfreight.com/radiat...ers-99704.html

Most likely the mechanic did not pressure test the cooling system and you did when you drove it. The heater core was weak and it popped as soon as it saw pressure.