A card that costs $100+, about once a day.
A card that costs $250+, about once a week.

I did once sell 4 dual-lands in a week, all in different transactions, but that was a fluke. Also, I was trying to sell them as a personal challenge.

Edit: This was a physical store, I assume larger mostly only stores would sell more.

Since I worked at a small store in the 90s, never really sold individual cards for all that much (or even had expensive ones) but large orders that were over 100? A few a day. A lot near the holidays, board games are expensive and the go to gift.