A friend is looking at purchasing a van that is in really good condition but the mechanic reports the engine has a leaky valve seal and it would be around 500 - 800 dollars for the fix.  I don't have much more information on that and apologize for that, but it is what it is.  

My question is, does anyone know what valve seal this may be?  What are some repercussions of driving it to get it home?

Appreciate your time,