Increasingly, dealers sell on internet auction sites like eBay or Delcampe.
Dealers also vary in the type of material they handle, ranging from a general stock to highly specialised firms who only trade in particular countries or topical areas.
The Stanley Gibbons catalogue is an exception that, in principle, still shows the price at which they would sell a particular stamp over the counter at their London shop, or by mail order, if they had it in stock and it was in the exact condition outlined in the notes at the start of the catalogue.
In practice, many items are not in stock, or are not in the same condition, or the price at which the stamp is for sale differs for some other reason from that shown in the catalogue.
The Gibbons catalogue was originally published as a penny price list in November 1865.