I'd say CA hardware and accessories. The only retailer I know of that actually has anything in stock is 007. Get some toys for them too, like rails, scopes, tac lights, everything. I'd just like a little bit more choice than just one retailer, even though 007 has fair prices for CA hardware.
Ooh! And stock regular upgrades, like metal slide and outer barrel for the KSC G19 for example. Just try to keep the popular items in stock, and the rarer items easier to come by, for the instances that people want them.