This trick is similar to the get a free house trick. First you must have the Investor perk in the speech branch of the skill tree and at least 500 gold can do this trick. When you go to a vendor you also need a storage container near the vendor. Warmaiden's in Whiterun is perfect for this trick. It's next to your home if you bought the Breezehome (not a requirement but for pack rats like me its a short distance to walk while encumbered if you want to sell all your old junk) and there is a large cabinet next to the vendor. Stand between the vendor and the cabinet. Choose the option to invest in his business and then the select "lets see what you can do with 500 gold...". Quickly cancel out of the window and click on the cabinet before the gold is taken out of your inventory. Put all your gold in the cabinet and close the cabinet. Now the vendor will have an additional 500 gold. You can rinse and repeat as much as you like. No gold will be taken out of your inventory and the vendor will never run out of gold to buy your goods.

Now you can save that perk that you were going to put in Master Trader and use it in something more combustible like Master Destruction.