I'm not seeing much of a use for this building as it's only worth it if in a turn you want to buy/sell a 'good', that you produce that the market isn't selling/buying. But for how much?
It doesn't state that it can exceed the normal buy/sell amount for the phase. Just seems a little useless.

 I think that you have misunderstood the use of the Taberna (either that, or I have!). As far as I am aware, the Taberna allows you to buy a good from the market that you can produce and which is available from that particular market. I've only played the game a couple of times, but I think the Taberna is useful as it may allow you to complete more contracts, use a commercial building, use the Redemptor and/or make a donation. Just my thoughts...

That's all well and good however if it allows me to buy goods from the market that I produce (and that the market is buying/selling that turn) then i see no need for it as I could just buy it normally.
Going by what you've said then I believe that maybe I can buy an additional 1 good that I can produce (going by the little '1' on the building tile). Which to me makes a lot more sense and a decent game mechanic.

However I could and most likely am wrong, I just have some kind of tactical blindness over the issue.

Cheers for helping.

 you cannot buy goods that you can produce unless you have the taberna

Ohhhh, oops

Cheers for pointing that out.