I manage and build websites for a living, and so I feel I can speak with some authority on this topic. If you like for your customers to enjoy shopping on your site, and doing business with you, keep your inventory information accurate. I have tried to purchase a number of dealer items in recent months, that show in-stock on the site, and I end up finding out the item was already sold. I cannot tell you how frustrating that is, to hear over and over again. Most of you I've never done business with in the past, and this is not a good way to start.
Dealers often absolutely refuse to deal on price, or to budge at all when it comes to working with customers, expecting the most from us. At the very least, take the very small amount of time to mark an item sold when it's sold. Most of these sites are awful and poorly designed to begin with, to be honest, and I rarely see such a disorganized market as is WW2 collectibles. Anything that can be done to improve the customer experience is going to make a big difference, and simply marking things sold may in fact put you ahead of most of your competition it would seem. It's that bad.