When purchasing items during lag, it may take over 30 seconds for your money to disappear and the items your purchased to appear in your inventory.
I bought a bunch of potions, walked away, talked to another merchant, did some other stuff, and about 30+ seconds later I finally got the potions I bought.
If I didn’t know that my original purchase of potions had actually gone through and it was just lagging, I could have easily tried re-buying those potions over and over and wasted all of my money on more potions than I actually needed.