This is what made me think: "Hey, I'd like that convenience on the CiC-Server, too (which I do consider my home-server ^^). To not have to run from trader to trader and check their inventories if you just want to buy a big backpack or a certain kind of ammo for example would be amazing. Sure, after the Server will be running for a while the Traders will have all kinds of stuff, but every trader will have other stuff and there will be no general guarantee that you will ever be able to find the particular stuff you are looking for somewhere.
AND stuff is organised in categories there, so you don't need to "scroll and concentrate" anymore

I'm not sure if they had vehicles in there also, I think they didn't have them in tbh. But never mind vehicles, the rest of the possibilities of that "other trader" would be still worth a one-time-donation (like in a2).
I know how hard it is to keep the servers running as a gaming-community, and every donation helps. So why not "motivate steady customers?" was what I thought

The existing traders would still need to stay, of course. Because "forcing" people to donate is not very nice. So the only thing that donating would bring here is "convenience".
On your A2-Server it had felt the same. Sure, as a non-donater one couldn't sell DMR's for example, but still, one could sell other weapons to gather money. So it's not "pay-to-win", that is what I liked about that idea.