Buy with an eye on design, form construction and material and keep in mind its place within its market.
Whatever you collect, look for the rarest and best you can afford and you will rarely go wrong. For instance, you’re better to spend $200 on one GREAT stamp than pay the same amount of money for a whole collection of thousands of common ones.
There will always be good money for good stuff.
(Shown here is the penny black. The Penny Black was the world's first adhesive postage stamp. It was issued by the 1 May 1840)
An interesting fact
Tiffany lamps were always a rich man’s luxury originally selling for between $30 - $750, but today they sell for $1000s into the millions!