A couple of things.

Looking at receipts should occur before purchase not after as should taking the car to a shop inspection.

Did you buy this car from the actual long term owner or someone that buys cars, cleans them up for resale but claims they are an "enthusiast" ? The latter is really a dealer but skirts the law / ethics by calling themselves an "enthusiast".

Before buying anything from an ad, I always run the persons phone # / e mail / name through a web search. Since the web is forever, I see if someone selling lots of cars but advertises in the " By owner " section of Craigs list they are not being truthful / ethical right from the start. The gray area would be if they clearly state in the ad that they buy and tidy up cars as a hobby / extra $, in this case they are being honest.

This person didn't "sell" you the used item , you made a decision to "buy" it. It is the buyers responsibility to make sure the item is suited to their needs / expectations.

There is a head gasket sealer called "Blue Devil" ( other brands have popped up recently ), it works , at least in the short term, I have not been around an engine that has been run with it long term. Look up the chemical " Water Glass" or "Sodium Silicate" . Look carefully at the rad cap area for white stains then look at the coolant for a pearlescent reflection.

Bubbles in the coolant generally indicate combustion gasses leaving the cylinder and entering the coolant. There are chemical tests for CO in the coolant, but I find pressurizing each cylinder and looking for bubbles to be more accurate as it narrows down to the actual cylinder. This helps when the engine is taken apart and one must decide if the gasket or head is bad.

Coolant bubbles can occur if coolant flow is poor ( water pump impeller losing fins. ) or if the cooling system had just been filled and air is still coming out of the system so the original diagnosis might not have been correct.