Update: Buy tab implemented! It's not active yet because it needs some polish but it works in principle! Should have it ready by tomorrow evening.
This is what it does:
Step 1: enter something into the searchbox (or shift-click an inventory item) and press search
Step 2: the addon makes a multi-page search and lists all results sorted by unit buyout price
Step 3: you select the auctions you want to buy from the list and press "Buy selected"
Step 4: the addon makes another multi-page search and buys your selections if it finds matching auctions (i.e., same price and same stack size)
I think this is the best that can be done with the vanilla API.