Those merchants who spam B>Scrolls, practical to sell to?

They're cheaper in FM entrance because you don't need a store permit. When buying/selling stuff you always have to keep track of your costs i.e. permit/hired store, scissors, potential scrolls, amount of time taken to buy/sell, etc.

The discussion was about buyers, not sellers, but I disagree with your evaluation of sellers.

I'd argue that most sellers, even those with permits, don't have the insight to deal figure in overhead.
It all really comes down to that last bit: time.

If someone is selling in the FM entrance, they know that there's little chance that they'll make the sale and simultaneously acknowledge that they have to be present to do so. Shop-owners, however, feel less pressure because they can set a price, accept bids, and adjust the price to make the sale.