Well, it's been a long long Sunday, and there have been more obscenities uttered than I care to admit, but I've managed to get the Vendor Interface working as I wanted it (mainly) finally.
The interface uses the data from the server to show available items for that vendor and shows (currently the basics, but has access to everything needed) information to let you decide what to purchase. When you double tap the price-tag, it creates a new item and places it in the player inventory as well as making the required changes on the server. Everything is handled via nice callbacks, so if the server doesn't know about it, it doesn't happen on the client-side either. That should sort out most network latency I hope.
I had hoped to get a bit further given my limited weekday time, but this just took longer than I expected it to. I'm still having the odd "frustration" with Android where something just doesn't, for the love of anything, seem to want to work the way I think it should. I spent around an hour today running down a side-quest trying to get a particular view to fill the width of the screen nicely. I've managed it without much hassle in all the other places I use this functionality. But here. Nope. Nowai. kthxbai.
Anyhow, good progress. With some luck, through the week, after work, I'll get the code tweaked to handle loot and quest rewards too. Hopefully all this works as intended with that.