What is the difference between 'actual' and 'potential' interaction between buyers and sellers? In the book "Microeconomics" by Pyndyck and others, the author(s) define a 'market' as a collection of buyers and sellers, who by the actual or potential interaction with each other determine the price of a product or a set of products.

What is the difference between these two terms that the author(s) have taken effort to differentiate?
The traditional representation of a market, with the curves of supply and demand reflects potential transactions: for each price, we can derive the quantity that would be offered or asked. The actual transactions only occur at the market price, but ignoring all other intentions is too restrictive to define a market. 

For example, stock exchanges often give the possibility to agents to express their intentions (that is,  the quantity they would offer or buy for a certain price) about a particular security, which will form the bid (supply)  and the ask (demand). Even if the ask and the bid never meet, there is still a market as people are willing to exchange goods, though they cannot find an acceptable compromise.