A volley is a mutually beneficial action between two players. It consists of buying a specific ally over and over until it reaches a certain point that is agreed upon. This benefits the person who keeps the ally because it gives them more plunder bonus if they are not max plunder, and it also brings the ally up to a value where it will not get bought if they are looking for a keeper. It also benefits the other player as every time an ally is bought from you, you make money. The higher the value of the ally that is bought the more gold you make from the sale.
Whilst you don't want to sell all of your allies just to make money, you can always sell them and buy now ones to make a little gold... this is more difficult than volleying as you will need to find a different buyer just about every single time, plus there is the annoyance of having to search for an appropriate ally to buy that will be good enough to sell off again.
