Any purchase made for wor is tax deductible...whether you purchase it at a 2nd hand store, a garage sale, online, etc.
Get a receipt from the individual you are purchasing the product from or make yourself a receipt.
If I purchase something online, I just have the person I'm buying it from send me an email stating the price and the item description. Or if you buy from Craigslist, just print off a copy of the ad!
