If the town was within 10km travel distance that every peasant (allegedly) adhered to in medieval times, and the peasant had a whole cart with many sacks full of grain that he wanted to sell, how would he go about doing it?

Assuming the market was the only place in which trade could occur under the watch of the lord of the land, what was the procedure for selling things at a market? What if he was only trading a small commodity - eg. he had an extra pot or chicken to barter, and he's willing to spend 8 hours going about it (before the 2 and half hour walk back to the village), because the town might have more unique wares than any nearby village.

If he was selling for coin, how would he know the coin was not counterfeit? How would he know that merchants weren't trying to sell him a bridge, when it came to the value of the coin he was given?