So I went to Riverwood to sell my Potions of Invisibility at The Riverwood Trader.

An Ancient Dragon attacked and I killed it. I sold 6 Potions of Invisibility and 'Waited 48 hours'

Another Ancient Dragon attacked again. I killed it again. I sold 6 Potions of Invisibility and 'Waited 48 hours'

An Ancient Dragon attacked again. I killed it again. I sold 6 Potions of Invisibility and 'Waited 48 hours'

I've did this 12 times in a row now. Killed an Ancient Dragon, sold 6 Potions of Invisibility and 'Waited 48 hours'

Why do these Ancient Dragons keep appearing at Riverwood? I am getting fed up killing them.

I still have over 800 Potions of Invisibility to sell. If the Riverwood Trader guy didn't have 11,750 gold I would just go somewhere else.

Is it a bug that these Ancient Dragon's keep appearing at Riverwood? Is there a way to fix it?

Do something else- don't sell the potions, and wait for 48 hours. Or leave Riverwood and return after 48 hours.

It's possible that you found a quirk of your specific hardware/software configuration that creates an Ancient Dragon whenever you sell 6 of those potions at the Riverwood trader, and then waiting 48 hours.