What are the symptoms of the game? My guess is the mpu is fine, most likely the daughter board ROM has broken cold solder joints. It's a very common issue in the 80b games.
I just fixed a Genesis for a customer that appeared dead. It won't boot if it can't read the ROM. I used the piggy deux bypass board and the game is rock solid now. I also did the grounding on the pcbs and the lower cabinet grounds.
The piggy deux is around $45 incl shipping.

If the rest looks nice i would pay $500.00 for it

I would pass......... it's a Raven. Lots of problems can arise on this system..... lots of connector work,ground mods, and pray the soundboard works. The guy selling it repairs pinballs and would rather sell it broken, that outta tell you something.

Worst case it would need a new mpu there $200.00 the ground mods are easy to do, I have 3 Gottliebs in my collection and after upgrading a few things they have been the most trouble free out of all my games. the only thing that would hold me back from buying it is if somethings missing or broken that you cant buy new, pinball resource will have anything you might need for it, I played a really nice dialed in Raven and had a lot of fun playing it, usually they need the flippers rebuilt and dialed in to make the ramp shot.

It's AT BEST a $600-$700 game fully working. Keep that in mind when considering the parts you'll need to buy and the time you'll have to put in fixing it.