I've been thinking about this very same topic with my Surf 'n Safari. I've installed almost all LEDs on it, cleaned most of the playfield, but never did a full teardown. Still has some dirt trails on the plastic slides and the shooter lane, etc. Everything works on it except for one of the slingshots occasionally triggering a few more times than normal. I also installed a cliffy outhole protector because the wear was nasty in this one spot.
Basically, I only did a partial shop job on it, and I'm not sure if I'll do a whole lot more on it.
I tried selling it last year but got a sour taste in my mouth after a potential buyer kept tire kicking me. He was saying his "friend" had a way better Surf 'n Safari, mine was a "below average" example, he would only ever pay X amount of dollars for it, etc. Dude, just say you're not interested and move on.
The point I'm trying to make is, if you're willing to be patient and also deal with tire kickers here and there, you should be able to sell your machine as is.