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. Some traders add a fixed amount to the base price of rarer goods (the Trademaster's Gem costs 10 silver where its base price is 1 silver).

Every item you take lowers the stock, so the next one is dearer. Take a whole shelf of 20 and the last one costs about 4.5 times the first (the square root of 20).

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

They can. A deal needs the trader to be able to pay you. Every night each trader gets its daily money (shown per tier on its page) and trades a little with the outside world to drift 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.

A Town trader marks some goods with a Likes label. Each one sold adds to a Likes bar; you get a Like 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.