I purchased an Omega Seamaster Planet Ocean Chronograph off Watchuseek which was in excellent condition to begin with but I still had to send it in for service for peace of mind since I had no idea what its life had been like prior to my purchase and I wanted to make sure it was in good working condition in spite of what the seller might have said.
They replaced several internal components that, to my eyes, look fine. They also replaced parts of the bracelet as it would not stay locked. They cleaned it up and slightly polished it up but it was in such near-pristine condition, they really did not have to do much to it externally. I couldn't tell much difference between before and after but I definitely could tell they did clean it up.
One thing they did NOT do is replace the bezel insert which does have a slight scratch in it at the 30 marker but it's not really noticeable or significant unless you're OCD. That's OK as I figured I'd knock it against something anyway like I've done with all my watches. So it's no big deal.
I brought it in to the Omega boutique at the South Coast Plaza in Irvine, CA and they sent it to the Omega service center in Oregon or Washington (not sure where it is) and I had no problems with the work. The watch came back sooner than they said it would and it came back in better condition than when I sent it. Adjusted and tested with timing and pressure check proof. So I was completely satisfied. I paid the generic $750 price I believe.