Any Hero has completed the main quest without buying anything.
I recommend doing this on a silver campaign after a bronze campaign. After doing bronze, silver will be quite easy. Potions will stack higher and higher, and you will have no need of buying potions from the merchant as readily as you were in bronze. Also, it probably is possible to just go to your bronze campaign, buy potions, and load your silver campaign, causing no harm towards getting this achievement.
Note: Buying from the merchants who sell the potions and equipement should only affect recieving this achievement. You can buy mounts and use the blackmith at will.