Not exactly a sutler in this blog's usual definition, but close enough to win some spotlight as a vendor. Battlesim.com is more a service provider, hosting events for historically oriented airsoft groups, particularly in the WW2 and Vietnam eras.
The site gives a nice succinct description of Battlesim.com:
BattleSim.com LLC is a company that services gaming communities and gaming players with a primary focus on "war" or "battle" simulations. These simulations range from airsoft events to PC gaming.
It's an interesting concept that combines reenacting vendor (they also sell stuff) and host roles into a service company. The site is worth looking at, if to get a feel for what Battlesim.com does. The forums are an especially interesting stop.