I have a few Amzn returned sets that are partially opened which I want to unload on eBay as used sets.  There may be 1 or 2 bags that were opened but most of the bags are still sealed.  Some of the sets are missing a few little pieces like 1x4 flats, parts of the base that nobody can see once the build is completed.  I can easily replace them with pieces of different colors.  But I do not feel like placing a bunch of Bricklink orders to replace something that is not visible at the end.

How much of these should be disclosed in my eBay listing?  Do I need to list all of the small pieces I replace with different color ?  In your experience, are the used set buyers so anal they would complain if I replace 1x4 red flat with a 1x4 grey?

99% complete. 13 out of 15 bags still sealed.

Price it somewhere between used complete, and new sealed.

I've had to play this game many times, and I've never heard anything from buyers.

I do similar.  In the description I tell how it is in new condition but with opened bags.  The rest of the bags are still sealed in new condition.  I start the ebay auction with a used price and let it go from there.  Right or wrong, I let the community determine the value.  So far it has worked well.  I will also make it as clear as possible exactly how complete it is.  If there are minifigures or not (which is usually not if a bag has been opened). 

I prefer to NOT reuse opened bags.  I received an ebay auction that was listed as new and had all three LEGO bags for the set.  I was very excited to receive the set in new condition (how it was advertised) until I realized there were no minifigures in bag #1.  After examining bag #1, I discovered it had been opened and resealed.  That's just deceptive in my book.  If you're going to reuse the bag, use a rubber band around it or something so people can see it has been opened.  More annoying was discovering it was missing a few pieces to boot.

remember many buyers don't read past the title

so must state "incomplete" or "99% complete"

I'm a fan of full disclosure; tell all you know.  As a frequent buyer of used sets, I really like it when the actual condition of the set is described.  Things like "Displayed only" or adult built and displayed speaks for itself.

If it's a pricey set I always need the seller to explicit state condition (items like yellowing of white pieces is a big deal; but often not mentioned on older sets even if they were displayed only)

remember many buyers don't read past the title
so must state "incomplete" or "99% complete"
I'm a fan of full disclosure; tell all you know.  As a frequent buyer of used sets, I really like it when the actual condition of the set is described.  Things like "Displayed only" or adult built and displayed speaks for itself.
If it's a pricey set I always need the seller to explicit state condition (items like yellowing of white pieces is a big deal; but often not mentioned on older sets even if they were displayed only)

I had someone purchase a new, but box less set the other day. They contacted me asking me to ensure I got it to them by a certain date (no biggie, as it was set to make it there a few days before).

The kicker was “it’s for my son’s birthday.” So I double checked with them that they knew it was no box. (Wasn’t an issue, and yes they read the title and description). But you just can’t be too careful sometimes.

14 hours ago, Alpinemaps said:

I had someone purchase a new, but box less set the other day. They contacted me asking me to ensure I got it to them by a certain date (no biggie, as it was set to make it there a few days before).

The kicker was “it’s for my son’s birthday.” So I double checked with them that they knew it was no box. (Wasn’t an issue, and yes they read the title and description). But you just can’t be too careful sometimes.

That's brilliant...give the kid only 1 bag per birthday

saves a ton