Traveling vendors are determined each week (or day). The total can be 0. Duplicates are allowed: two selling almost the same thing create competition and push prices down.
Roll 1d20 once per settlement. The state shifts prices and stock by category, how many traveling vendors show up, and which ones are more likely.
Weighted table, adjusted to the selected size: everyday trades take wide ranges, rare shops narrow ones (and grow in big cities). Roll 1d100 for each variety stall shown in the sizes table. Reroll if a duplicate comes up.
Roll 1d20 at the start of each week, before the traveling vendors: some events add or remove vendors.
First determine how many with the die from the sizes table (min. 0), adjusted by the state and the weekly event. Then roll 1d100 for each one here. Duplicates are valid.
Roll 1d12 per vendor for its price level vs. the book's base price. If two or more sell the same category, also apply −10% for competition.
Roll 1d6 per item (services are always available). The settlement state can add or subtract from the roll. Then 1d6 per vendor: on 4+ they have a special piece — roll 1d6 on their category's table.
One roll on each table per vendor. The attitude changes the haggling DC. The rumor's truth is for the DM only.
Compare your Persuasion (Charisma) check with the vendor's DC (shown on each card):
Use each card's calculator: enter the total result and see the negotiated prices. Peddlers and charlatans give a lot; temples, enchanters and famed smiths, little.
Base prices from the D&D 5e equipment tables (gp = gold · sp = silver · cp = copper). Exotic, magical or dubious items use estimated values. Adjust to your world freely.