I'd have to say there is no traditional market for soul 45's here. First, the truly top northern records are always going to go to the UK anyway; if any US buyers want them they don't even bother bidding on them when they're listed on ebay. Soul collectors here may trade with one another, which makes the value relative. Lower end records, like the Flirtations "Nothing But a Heartache" or something like that, sell on Ebay for just a few dollars (like...a quid, right?), and I always suspect that the British would pay more than that if not for the shipping cost.
I never buy individual records, always lots (and always cheaply). In the spring and summer it means going to as many yard sales as possible, asking the homeowners if they have any records (most people, here in Pennsylvania, still have no idea that vinyl is "back", and if they do want those old records in the basement it's the LPs they want, not the 45s). In the fall and winter I'll buy lots on ebay, avoiding record sellers and looking for offerings from the uninformed. Most of those sellers don't offer overseas shipping, but I'm not sure it matters - I've won auctions for "peanuts" where good soul records are sitting right there in the auction photos but no one seems to notice or care. Otherwise, estate sales and auction houses are good prospects as well, but as with the yard sales you do have to stay one step ahead of the other record hunters. Often, though, that competition is more interested, and knowledgeable about, classic rock LPs than they are about 45s.
I love soul, and I think the British fans have rightfully recognized the best of it, but my own collection consists mostly of the beat up copies - I sell off the best stuff so the hobby doesn't end up costing me too much money.