Typically, there are four places to consider buying used power tools: from rental locations including Home Depot, from tool repair shops, from Craigslist or private owners, and from Pawn Shops. All three have pros and cons, but for the sake of convenience of selection and in having a shop that can back what they sell, a pawn shop can typically provide the best balance of price and selection. In the Treasure Valley area, one of the largest selections of tools can be found at Vista Pawn.
There are two types of power tools that can be considered when purchasing: cordless and A/C or D/C powered. When looking at cordless, in addition to looking at the basics of purchasing , there are two tell-tale signs of the reliability of the unit. First, make sure whenever possible that a brushless motor is bought. Brushless motors last years longer and require less power draw on the battery. Second, bring a voltmeter or have the shop lend you one and have them give you the battery in a full charged state. Check the voltage of the battery to validate that the battery is still able to come close to a full charge; for instance, a single cell lithium battery may be full discharged at 2.8 volts and full charged at 3.3 volts, as an example. If need be, and this is the beauty of a pawn shop, come back and make the purchase later in the day after checking the reference settings of the battery on the manufacturer's site after selecting the tool you want.
For corded tools, you will not typically find brushless motors since power draw is not as important. Other aspects of the tool's longevity that are important are lubrication, checking for metal shavings in the gear assemblies or connections, and simply checking the functionality of the unit by turning it on at the shop. For some equipment that is more than just a simple tool like a drill or saw, it may be a good idea to have other tools available such as a torque meter and voltmeter, to ensure the tools are up to specification. A good source for finding manuals not available easily is here.
Finally, whenever purchasing a tool, ensure the seller is reputable, and will stand behind the product they sell. Vista Pawn is a good starting point, and generally, most shops like them will try to earn your business by honesty and a generous return policy. Avoid shops that are looking to make a quick dollar and have no return policies, as they are likely running in to cash flow problems and cannot afford any other policy than a final sale approach. Haggling is important when purchasing used gear, but at the same time, don't forget that their is overhead and convenience to the customer when purchasing at a pawn shop. While you may pay slightly more than a private party sale at a pawn shop, you have recourse in the event that the tool breaks down the road. Ask for a 6 month warranty thrown in at no charge, as most reputable brokers will provide that without a problem.