I've noticed that in at least some cities, the smiths will purchase trade goods for the same price as the merchant. I haven't verified if this is the case in every city. Apparently, the smiths aren't Guild members, but they still set their prices based on the same formulas.
It's still impossible to purchase most trade goods in decent amounts when one is on the outs with the Guild, but one can sell one's current load at least.
It would seem to be the best strategy for hack-and-slash oriented players to take a spice delivery mission, sell the goods and move right into hunting bad guys for a living, bypassing the trade elements of the game altogether. One misses a lot by playing this way, but to each his own.
Like Shmonkey, I played one game in which I ripped off the Guild for over 20k and went on a spending spree. Unfortunately, poor Feodoric was jacked by Kerghit raiders a few days later, and is now begging for enough denars to purchase a limp horse for the ride back to Zendar.