Best bet is to ask on COTH http://www.chronicleforums.com/Forum/ . The magazine/website also does advertising but it's not really worth it unless you're going to spend for a big ad and they're not cheap.
There isn't really *one* site or even magazine for eventing in the US as it's a HUGE geographical market. People do buy across the country for really special horses but more often they go up and down the two coasts. (There are pockets of eventing in the interior but mostly lower levels - the really big events (Rolex, Jersey Fresh) are in the East but the West is growing.
I'd say much more selling gets done in North America through professionals. There are fewer dealers, as such, but most coaches do some buying and selling, at least for their own clients, and have a network of contacts. It depends on what you're selling. Lower level horses would usually have local-ish ads, upper level ones would be through a professional and might not ever be advertised publicly at all.
hi, I have warmbloods, but did sell a warmblood x who was an advanced event horse turned pure dressage on a website www.warmbloods-for-sale.com hope thats some help, they maybe more links to other US websites on there