Used WMF Stainless Tableware Condition Issues
While some of the issues discussed below apply to all stainless steel tableware, this discussion is primarily aimed at condition issues surrounding vintage WMF Cromargan stainless made in Germany after WWII.  WMF has since patented a process called, "Protect," ostensibly to reduce all forms of "aging" in stainless, but it has come too late (2009) for those of us with vintage sets of WMF.

I would urge sellers of used stainless to carefully examine the pieces they have for sale and honestly describe, better yet photograph, any flaws a piece may have, especially any flaws that cannot be fixed with a light buffing and/or a good polishing.__If you have a damaged piece, you need to resign yourself to the fact that you cannot get full price for it and be happy if you can find a buyer for it, at all.

-  Introduction    Backstamps and Blade Marks 
Issues of Condition

Used stainless can be expected to have scuffs and scratches as a part of normal wear.  These can typically be buffed and polished away, if one chooses, so they shouldn't affect the saleability of a piece.  However, because I'm filling out a set of WMF "Line" flatware that is over 40 years old, I wish people wouldn't buff the pieces they put up for sale because, in an old set, the buffed ones stand out like a sore thumb.  Also, if the original finish was satin, rather than glossy, as is the case with Line, too much buffing can destroy the original satin finish altogether.  It was not glossy to begin with!

An issue that is literally deeper than light scratches is gouges, meaning a scratch that cannot be buffed away, or dings, meaning a lost bit of metal that cannot be replaced.  In the case of holloware, dents (depressions) are a major issue because it takes a skilled hand (e.g., a jeweler) to pound them out at considerable expense.  This problem also applies to hollow-handled knives, where the consequences are even worse because the damage cannot be hammered out, so any dents or depressions ruin the piece.

Another unrepairable defect is pitting.  Pitting represents a corrosive loss of metal that cannot be repaired it's too deep to buff away so any pitting severely reduces the value of a piece.  Unfortunately, older WMF stainless pieces, especially the blades of knives, are prone to pitting.  I have, in fact, sworn off buying used knives, at all.  I'm just going to live with the ones I have.  Below are photographs of examples of pitting, mostly on knife blades.

Stainless may be resistant to rusting, but it's not so tough that it can stand rough treatment.   To avoid pitting your stainless, never soak it.  Especially, do not soak it or even wash it with chlorine bleach, cleaning agents containing chlorine, or along with other metal utensils.  Avoid allowing it to remain in prolonged contact with acids, such as tomatoes, tomato sauces, or fruit juices.  To avoid getting "rubs" on your stainless (and marking your dinnerware), wash your stainless by hand, not in the dishwasher.  If it's expensive stainless, handle it as gently as you would silverware.  A good polish for improving the appearance of your matte stainless is Purargon, manufactured by WMF.  Glossy stainless can handle a stronger polish, such as Wenol.  I emphatically do not recommend buffing.  It may make a piece "look good" for table use, but you can write off the "collectible" value of anything you subject to buffing with a power tool, especially if you wear down the backstamp.
Examples of Pitting on Some WMF "Line" Dinner Knives

Pitting on Other Pieces

On the handle of a WMF LINE soup spoon:

On the front of a WMF LINE dinner fork:

On the back of the same fork:

Depression of the Handle of a Hollow-handled Knife

End view of an undamaged knife:

End view of a depressed/crushed knife handle:

Excess Buffing
A knife blade with the maker's mark almost obliterated in an apparent attempt to remove the pitting, which is nonetheless still visible (this knife was sold to me by Replacements Ltd.).  Excess buffing is one reason I would rather buy used pieces  from amateur sellers (e.g., on eBay or Etsy) who have not buffed them, provided they have fully and accurately disclosed the true condition of the piece. 
Sadly, I have occasionally found sellers who are less than forthcoming about defects in the used items they sell, so do ask questions, and ask them specifically and keep copies of the interchange as proof in case of a later dispute.