We sometimes feature some older games on our blog. If you are interested in them but are worried that your too-new (or too-old) computer might not run them properly, then Good Old Games is an interesting alternative place to shop. Good Old Games offers a 30-day money back guarantee, so you can try games pretty much risk-free.
When you buy a game from, say, Amazon.com, they do not care whether you can get your game working or not. All that matters is whether you received your purchase. This is fine if you have purchased (for example) a Collector's Edition of a game that comes with extras such as books and posters. Also, Amazon.com has more restrictions on which countries they can ship to.
What Good Old Games does different is they will work with you to get your game running. In the event that reasonable steps fail, they will give you your money back. You can read all about the Good Old Games money-back guarantee on their website.
If you are looking for a specific item that might be out of stock now, try Amazon.com, which may have resellers willing to part with it cheaply. If you are looking for a re-packaged older (or new) game with programmer support to make sure it is up and running, definitely try Good Old Games.
Additionally, Good Gold Games is DRM (Digital Rights Management) free. That means, unlike a vendor such as Steam, you are not married to the service provider and dependent on them. You own a copy of the game on your computer, and there is no activation or online connection required to play.
Less than $5 USD
Finally, Good Old Games frequently has sales resulting in games being sold at less than $5 USD, sometimes even less than $2.