I repair and sell mostly the mid range Stihl saws such as the 1127 series (farm boss, etc) and a few of the MS250, etc.
I can pick these saws up fairly reasonably since they are so labor intensive to repair that most people won't. They are also very common, well known and desirable in the market.
As far as cosmetics, I have never painted anything on any chainsaw ever. That said, the extent of my "detailing" is using a black sharpie to enhance the Stihl logo on the recoil cover, etc.
I recently picked up 10 new aftermarket bars and am going to try to see if the saws move better with those.
When buying saws, I always look with suspicion at saws that have painted parts. I would rather have them original so I can better judge wear, etc.
Anyway, I sell the MS290 and similar for about $225 to $250, the MS310 for about $300 and the 039/MS390 from $320 to $390 depending how they're set up with bars/chains/sprockets.
As stated above, cosmetics seems to be a major determination in saw value and the other is bar length.
Regarding changing parts, I replace anything that's defective. I don't change any parts that are working. For instance, if the carb needs a kit, I put one in. If it works as is, I leave it. I use a lot of used parts and also use aftermarket parts extensively. If I have put aftermarket parts in the engine such as a cylinder or piston, I make that clear when selling it. I try to give full disclosure on my saws and often educate buyers on a number of issues which they seem to appreciate. I try to make a little money on every saw but in reality, it's a hobby. I don't think I could make a living at at. I figure I make at least a couple bucks an hour on average. lol. I do it for fun, not a job. The key (this is really important) is to buy cheap. There is a cap value on a used saw no matter how much work one does to it so to have margin, you need to create that with the initial acquisition.