Hi Mitch,
First, mint is a term way over used and subjective to each person's definition.
Example, Tom Mayo takes a few folders to a show. They get handled for a couple of days. Technically this would no longer be New In Box (NIB). In fact some would consider these nothing more than floor samples that should be sold at a vastly reduced price.
Perhasps two blades accidently hit each other, now they are dented and dinged. These knives should be reduced even more.
Are they, of course not. Are these knives truely mint, no.
Mitch you can't have it both ways. Those people who insist on knives that are truly NIB, then cannot expect to be able to go to shows and handle those knives. Taken to an extreme, it would get to the point where I could only let you handle the knife if you had pre-paid to do so.
Most dealers I know only deal in "mint" knives, that is to say unused. My trade in policy states that the knife must be in the same condition that I sold it to you in. Most collectors of custom knives have a few EDC's and the other knives in their collection, sit in a safe or on display somewhere. As such, the knives are in "mint" condition and are pre-owned.
Additionally, most dealers I know will tell you the history of the knife, to the best of their knowledge.
An interesting point here is that most of the most sought after and high dollar knives that you find for sale are pre-owned. Loveless, Herron, Scagel, Moran, Walker, Lake, Horn, etc. are pre-owned. The fact that they are pre-owned has no negative effect on the price of the knife.
Think about other collectible markets, like Antiques, Art, Cars, Coins, Stamps, etc. Almost every one of these at the top of their category are pre-owned. In the case of the cars, most have been driven!
Returning back to custom knives. Most knives that are over say $600 are never used, these more often than not find their way into collections. This is not to say that on occasion someone will not slip a hard to get knife into their pocket to show those who appreciate it. Also, their are people who bought the knife to use....and use it they do. Not to many mint condition Scagel's out there. Some are in down right horrible condition. Of course these barely bring over $10,000.
92 degrees, brought up a good point about return policies. The Consumer Protection Act allows you the buyer, 3 business days to reconsider any purchase that you did not get to personally inspect the item before purchase.
No matter who you buy it from, maker, dealer or another collector. You have the right to return the knife.
Some dealers will try and hit you with a re-stocking fee. Generally, this is equal to the amount of profit they are making on the item. This is illegal. You should report any one doing this to not only the Post Office (if your package was shipped through the mail), but the Attorney General's office of both your state and the shipper's state. Formal letters can be sent to the Better Business Bureau and local Chamber of Commerce as well. If they belong to the Knifemakers Guild or the ABS, letter's should be sent along to them as well.
Dealers do trade amongst themselves. This is done for any number of reasons. More time than not, it is done to rotate merchandise. As one of the few Custom Knife Dealers who has a web site and set's up at shows. I have a two-headed monster to feed. As such I am always on the look out for custom knives to buy, pre-owned or not.
I will tell you that I get well over 80% of my knives directly from the makers.
As for trading "exclusives" I don't know of this happening on a regular basis. Most custom knife dealers have only a hand full of "exclusives" say 5 or 6, it that many. Far and away I have been involved in more "exclusive" custom knives in the last 6 years than any custom knife dealer in the world.
Personally, I enjoy seeing my LDC and Vanguard knives on other dealers sites. This is not to say that the other does not happen. I have had one of Larry's KnifeArt exclusives, the Viper by Brian Tighe on my site. Also, one of Josh's Triple Aught Design exclusives the P-38 by Mike Vagnino (Im pretty sure it was an exclusive).
Because of an exclusives limited production, most dealers are not key to trade these away. This is not to say if the right deal comes along they won't.
Having been involved in bringing over 60 exclusive knives to market in the past 5 years. I can tell you it can be a costly, labor intensive and time consuming project. Im sure Larry and Jay and a few of the other dealers with exclusive knives would agree.
Generally, speaking when you see an "exclusive" knife on another dealers site, it was pre-owned.
Personally, I think with dealers at least, it is a presumtion that some of the knives on the table are pre-owned.
As with everything else in custom knives, the burden is ultimately on the collector to do their homework prior to the purchase. I realize that this will take away from the pure fun of buying any thing that catches your eye.
As this "mini-thesis" comes to a close, I would like to point out and I'm sure this will come as surprise to no one. That I rather enjoy the "hunt" and the education I learned each time I considered buying a knife. I found going this route really increased the amount of pleasure I gained from collecting knives. As I grew tired of having buyers remorse at the end of the show because I did not do my homework.
One last thing, you know Mitch if you bought a copy of my book it would help you out a lot!
But that's a side track - I bet one of the dealers paid you off to side track me! 
