The selling of articles from place to place on foot is

This question is testing knowledge of the different forms of retail trade and how they are classified according to the method used to reach the customer.

Selling articles by moving physically from one place to another, on foot, carrying the goods to wherever buyers can be found, describes itinerant trading. An itinerant trader has no fixed shop; the trader's mobility is the defining feature of the business, and this form of trading is common with small, easily portable goods such as clothing, household items, and foodstuff.

The other options describe different trading arrangements. Exchanging goods for goods without the use of money is barter, which has nothing to do with the location or mobility of the seller. Selling goods through catalogues and having them delivered by post is mail order, which does not involve moving from place to place on foot. Allowing customers to pick items themselves and pay at a central point before leaving the shop is self service, which takes place inside a fixed retail outlet rather than on the move.

When a question describes movement of the trader (rather than the goods being posted, or the customer serving themselves), think itinerant trading.