Is there any? 
They say everything has its good and bad sides. So, what harm gun collecting can have for the gun market? For gunning in general? For the person who engages in it? 
What do you all think?  
I'm a pawnbroker/FFL. I talk to people every day who are in the market for guns. The thing that I've noticed is that more and more guns are ending up in fewer and fewer hands and most of those guns never see the light of day. I'm not talking about high-end double shotguns but simply what used to be considered "good" guns -- Remington 700 BDL rifles, Smith & Wesson and Colt revolvers, good old Wingmasters. These used to be the stock in trade among everyday gun users/owners. The hunter looking for a good deer rifle could always find a good selection of used Remington and Winchester rifles at any gunshop, pawnshop, etc. Today, the racks are fill of cheap rifles with parkerized barrels and plastic stocks. That's all today's shopper buys because they're cheap. 
The "good" guns aren't on the used racks because the collectors have snatched them up and squirreled them away. Many of these guys are in their 50s, 60s, and even 70s. Many are still in their prime earning years and the kids are out of the house so they can afford to buy guns and put them up. They're not trading them off or selling them. THey're just buying. Consequently, there are fewer and fewer good old guns available and all that's out there is the modern junk. 
Of course, in another 20 years or so, these same guys will be moving into the nursing home or passing on. When that happens, their guns will hit the market. Flood the market, more likely. Unfortunately, the upcoming gun buying generation won't be interested in wood and blued steel. They'll want hydro-dipped and camo plastic stocks, 30-round magazines and Picatinney rails. Wait a few years and you'll be able to buy a bunch of great old guns for a song.
ETA: Upon further reflection, perhaps the above actually only applies to brick and mortar shops. There's plenty of good old guns on Gunbroker, etc. Only problem is that the sellers put such ridiculous prices on them they never sell. They just relist and relist.