YEah it can often be hard to find a good velvet place.
Found an awesome tip on the Taleworlds forum. This is quite likely what Foxbyte meant but here is the full exploit.
Buy 1 unit of velvet and spice. (from anywhere you can get it doesn't matter what you pay) and keep it in your inventory at all times
Find somewhere where velvet/spice is cheap even if they don't actually sell it.
Sell them the unit from your inventory.
Ask the mayor to get a caravan of velvet/spice. As you sold them the good the game now thinks they have it. This is the trick.
Buy your seed unit of velvet/spice back.
Take the caravan to your destanation city.
Profit!
Repeat!
There are random places that have very low demand for velvet/spice all around the map but don't 'produce' them. Often they are right next to somwhere that has normal demand. Thus you can run caravan routes to adjacent cities/forts that are only 20 secs away from each other using this trick. Having found your cheap city you can then run multiple short caravans to all the cities around it and quickly make a fortune.
Going to try it out myself soon, sounds like a ridiculous exploit though
Then just have to stash a million in the bank and live off the monthly interest and not bother about trading again!
Edit: Just tried it myself, its clearly broken. Going Pskov to Navara, ie. about 10 seconds away with ctrl + space. I was buing at 40k and selling at 90k for a large caravan. You can make a million in about 5 mins. Then make another million. Then you can put a million in the bank giving you a monthly income of 140k to pay for troop costs, and spend the rest on pimping yourself out.