Just keep all the stock parts and you can sell the car stock or with the parts. If the buyer wants it stock then sell parts off separately. Hard parts bolted to the engine are harder to do that with.

Simple tasteful mods generally won't hurt a private sale but don't plan on getting much extra money for them.

 

Thoughts on mods done by say, Ruf? Still a "mod" like any other or is that a different class?

 

I'd say that's a different class. Haven't priced them, so I'm not sure what they're selling for. I'd wager they go for a fair bit more than a standard TT, and I'd be willing to pay more for a Ruf than an equally modded TT.