You can't have it both ways. Geographic diversity of participants is key to maximizing selection.
Of course that adds the problem of shipping costs.
Personally, if I could get ammo I need and use at a reasonable price + actual shipping, I'd strongly consider it.
Rather than come up with a complex escrow process, what if you setup a sub-forum or even topic for this? I've seen something similar on hand loading message boards, where a certain "karma" level is required to participate. The idea would be that people would be willing to buy and sell at their cost + actual shipping. It'd be totally honor system, but there are enough folks here on TSP I trust reasonably well, I'd wouldn't hesitate to exchange with.
So if I see I screaming deal on a caliber I don't necessarily use, I might buy a bunch - not to hoard, but to distribute with fellow TSPers.
MODS: can you advise if an ammo exchange/classified sticky topic would be permissible? The actual payment and personal info would be limited to PM of course. Just an idea.