It just occoured to me, while reading the thread about "what did you do before the internet". The internet has made some things more expensive. I'm not talking about everyday stuff you can buy off Amazon, I'm talking about rare stuff, vintage stuff. Car parts, for instance. If you were trying to sell an engine that was in your garage, you'd have to put an effort into selling it. Post some ads up on cork boards in local stores, take it to the local swap meet, tell your friends "hey if you know anyone looking to buy a 383, I've got one for sale." Last resort you'd take out an ad in the paper and hope it sells before you need to renew it. It took actual effort, and you were better off letting it go for cheaper than you were hanging onto it and putting effort into selling it.
Now, you post an ad up on Craigslist, and as long as your asking price isn't too outrageous, you can have it sold in a matter of minutes. There's no effort involved, and if you don't like the first guy's offer, there's likely 3-4 more offers right behind it.
Small, shippable parts, like air intakes... post an ad up on eBay or the EE on related websites, and you get not only local but worldwide hits. Your part is no longer just available to the local public that are lucky enough to see your ad, now you could sell it to someone in a city across the state, a state across the country, or even a country on the other side of the world. You can take the highest paying bidder, instead of having to worry if you'll find another person who is interested.
Thanks a lot, internet.