Everyone wants to buy low and sell high. That's how your shop in question would make the profit to cover the overheads and other expenses. If they pissed you off, just crossed them off your list and move on. Though I do appreciate your sharing here as it would enable the rest of us to avoid flipping to them, at the cost of suffering similar heartaches that you had.
On another note, even forummers do that sometimes. I've flipped some pieces, and I have had offers as low as 50% of the asking price. I'd normally reject them in a diplomatic manner, but some of them would come back with the reasons that I should sell at the low prices they offered. And subsequently after I closed the thread when the watch in question was flipped, some of them had the cheek to WhatsApp me to whine that I should have notified them before closing the sale as they could then up their offers. So, just take all these lowball characters with a smile and a pinch of salt, Adam.