Turns out my shell service wasn't banned from eve-central after all. The issue was a bug which made it look exactly as though it was -- complete with realistic timeouts on connection attempts.
Anyhoo, I'm trying to build a list of item types which are worth trading. My eventual plan, if I can get it to work, is to move items between trade hubs, which is a high-volume, low-margin proposition, so I've decided somewhat arbitrarily that I can ignore any item which Jita moves less than a billion isk worth of per day. We know The Equation can be off by at least a factor of 9, so I'm having it drop items it thinks move less than 100m per day.
Which turns out to be quite a lot of them. My list of items with market groups, minus blueprints, comes out at around 9700 items. Watching the process run, I'm expecting the number it thinks are worth trading to be less than half of that.