Each item has a base price and each trader a normal stock of it. The price of one is: base price x square root of (normal stock / current stock). At normal stock that is exactly the base price. Herm and the Trademaster add a fixed amount to the base price of some goods (the Trademaster's Gem costs 10 silver where its base price is 1 silver).

Every item bought lowers the stock, so the next one costs more. Buying out a shelf of 20 makes the last one cost 4.47 times the first (the square root of 20).

75% of the current price, capped at the item's base price. Selling also raises the trader's stock, so each next item pays less.

Yes. A trader only pays out of its own purse. Every night each trader gets its daily money (shown per tier on its page) and trades with the outside world, drifting back toward its normal stock.

Bags, certificates and Town Gratitude have a fixed price per item, but the game multiplies it by how many the trader has on the shelf: with 5 Town Gratitude in stock the first one costs 5 x 10 silver.

Januarius, Herm and the 12 Town shops pay Likes for some goods. Each one sold adds to a Likes bar, and one Like is paid each time it fills. Each item only counts for a set number per week, and each trader pays at most its weekly Likes cap. Both reset on Pride day.

Seeds come in packs of 4 (planting takes 4 seeds), and a trader does not offer seeds when it has fewer than 4 left.