I figured it was a 4 cylinder... had me thrown off for a bit though lol!
Another common failure on these is the vacuum switching valve for the EGR system. Actually maybe even more common than the EGR valve itself. Also often times an these parts can pass testing and still be at fault.
I hate "throwing parts" at vehicles without going through a proper diagnosis but unfortunately that's all I can do through the forum. But ANYWAY,
Don't let him give you "oh it's fine to drive" garbage like nothings wrong.
I hope the seller takes care of you and if for some reason you wind up stuck with it as YOUR problem feel free to PM me. I'll help with what I can and I wouldn't mind sending you a MAC code reader/clearer I have that would fit in a bubble wrap mailer to have for a week or so. I've been trying to help out who I can here and until someone gives me a reason to otherwise I'll try my best.
GOOD LUCK!