Because of their value, both for postage and for collecting, they can be used to defraud the unwsary and to fiddle tax.
Many serious collectors will spend several thousand pounds on a particular stamp. This means that they are ideal for tax dodges. Because stamp prices are entirely dependent on condition, a collector might flog all his fine condition stamps – worth tens of thousands – and replace them with poorer equivalents – worth hundreds.They can thus be used to launder cash, avoid death duties, smuggle money abroad. It's illegal but it happens all the time.
Experts point out that Switzerland has become an important place for selling stamps. It is possible to stick a handful of stamps on a stockcard, take them into Switzerland, sell them and deposit the receipts in a Swiss numbered bank account. In this way money can be safely tucked away, far from the prying eyes of tax authorities and other regulatory bodies.
At one time, in the sixties and seventies, when stamp values were rising, business owners bought vast quantities of mint stamps as a tax dodge. They wrote them off as business expenses – after all the tax people were scarcely likely to count the number of letters being sent by a business. In fact the price of stamps fell dramatically and anyone expecting a tax free windfall would have been bitterly disappointed.
Often stamps have been stolen. In the Second World War it was commonplace for the British soldiers to wander into German post offices and help themselves. About fifteen years ago when that generation started dying off, relatives would find bags full of contraband German stamps among the effects. Of course, most World War Two German stamps are worth next to nothing.
Anyone interested in the history of stamp collecting, should read Jack Shamash's book – George V's Obsession – A king and his stamps. Also, his book – The Sociology of Collecting. Both are available on kindle.