Remember reading recently, for the life of me don't remember where but it was respected author (at least by me) that around 1/2 of all the Lugers for resale were refinished, some better, some worse. Not sure if the number is 100% accurate, but that is why this site is a God send and the members have never been shy in "helping" a fella out.
No problem as long as seller is telling the true story, have several shooters/refinished and "any Luger is good Luger" so to speak. But I do agree with that author in that there are LOTS of folks not telliing the skinny about the provenance, and probably know they are not imo.
One more point I would humbly add (okay, two). They are:
Handle as many models/variations as you can before you purchase. Reputable dealers with true orginal finishes can teach a LOT, but once you've seen a true pristine DMW, or Mauser for that matter, you'll appreciate the effort those folks put into their products. Crisp barrel bands, or the crisp edge of an untouched sideplate, deep and clear proofs, fire blue where fire blue was, or straw for that matter, unfinished muzzle ends, etc. are all a part of determining what a model is worth, and worth to you.
And, read, read, read some more before plunking down the bucks. This forum is one VERY authoritive source but the list is huge of reference books, some by forum members themselves.
Hope this helps, and THANKS for this OP. GREAT additional info.