Naively assuming that the eBay Java SDK would be the easiest way to do this, I download it (all 55 megs). It unpacks to about 250 megs on my hard disk, most of which is documentation, apart from the 20 megs or so of compiled Java code that forms the actual SDK. Now, I expect that this documentation answers just about any question I might possibly have, only it seems that all the useful bits are spread all over the place... argh!
So now I'm going straight to the XML API and writing the thing in Python first, then I'll port it back to Java when it does what I want. I've got more done in minutes with Python and the XML API docs than I managed in over a day with the Java SDK. I can't imagine that it'll be *that* hard to port it back to Java, either... and doing it this way will result in a few hundred kilobytes of class files rather than over 20 megs to send to the client. Hmm.