It is a great idea John and I am sure you have thought through a business case for doing this. I take it you would be not just selling to the forum? That would mean, I suppose, either a commercial website or an ebay shop. Either way you will have to consider either carrying stock requiring a substantial investment with slow return. Or, getting a really good supplier(s) who will enable you to obtain the items ordered from you quickly to provide a reasonable service in delivery time to the customer. Doing it this way would inevitably cut the margins as you will not, I would assume, be able to by single items at wholesale at the same price as multiple items.
I have no doubt that most people on the forum would support the venture, especially as part of the reason is to fund the forum itself. However, if funding the forum is a prime issue, maybe you should be looking at it becoming a reasonable subscription for members.
It would be an interesting idea but one, I am sure you are aware, requires much thought.
As far as what would people buy? For myself, I rarely know what I am going to buy. Very often it is on impulse sometimes it is something I have been thinking about for some time and then the opportunity presents itself, case in point being the Japanese WWII Aircraft Carrier. I have had this at the back of my mind for some months and two things came together. One was the Group Build on Promodeller and the other was actually seeing the kit I wanted while visiting a shop while on holiday. Had I not seen it in the shop would I have ordered it online? Maybe not for a long time if ever.
Paints etc are always wanted by all modellers, it never ceases to amaze me that no matter what stock of paint I have, I still have to buy something new for each kit. Trouble is, you do a straw poll of members as to what brand of paints they use and you will cover just about every make. Is it possible to stock that sort of amount? Bringing us back to the question I started with. Stock or supply on demand. If you can get suppliers to enable you to work like that, it is the way to go. Minimal outlay for a lower return but not a load of dead money sat on shelves gathering dust.... sounds like a stash to me lol.
If you can do it, go for it. You never know, it could lead to a living.