I really do not know what to tell you as far as what you should do, but it could be that this particular place wanted you to buy it because they figured you would then need to bring it to them to have the work done.
Now I am not saying this is the case, but I would be suspicious of it at the least.
If the car seemed fine, and has only now had it happen, it could be inevitable that it was going to happen, or it could be a fluke.
The guide rails I believe are plastic, so they will become amber no matter what. and they do tend to get brittle.
Was the car using oil when you bought it? that is a sign that the valve guides are probably worn, because they let oil slip through.
When I took my car to my mechanic, he looked it over, checked fluids, listened to it, and drove it and put it up on the rack to take a look, and said it was all good. He did notice a msall tiny oil seapage from the head gasket area, but it was very dry oil, and the car has hardly used any oil between 3k oil changes.
7000 miles later, roughly 6 months down the line, I have had to have the tranny rebuilt.
Sure he probably could have pulled the pan and inspected it, but would I want to do this to several cars that I am only thinking of buying? and what would the car's owner think if you were to do that to their car.
There was no way to predict this happening on my car, and he drove it and the car drove great up until the one day, no noise of a break, nothing, I just lost 4th gear completely on the freeway, and then I partially lost 3rd gear, which would slip out occasionally, of course the first thing I did was take it to my mechanic, and had him open the tranny, only to find the clutch packs disintegrated int he pan in the form of black stuff.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.