In the early days of ebay, a friend of mine made decent spending money for college exploiting ebay mistakes.

He would search out items in high demand, but instead of searching for things like "Playstation PS3", he would search for "Paystation PS3".  He would purposely misspell items in the search to try and locate those items that no one was monitoring.

He would then re-list the item properly and make a decent profit.