Tips on buying a used drum set.

There are several places to look for used sets; begin your search by browsing online classifieds sites like Craigslist. These sites have the advantage of being hosted locally, which allows you to inspect the drum set in person before buying.
You also have to keep in mind; who is the drum set for? Are you buying it for your kid to give them something to do in the basement or are you buying it for yourself to learn and someday join a band? These are very important factors when choosing the right drum set.
 When buying a used set, you can't guarantee that the set has been well taken care of. The first area to inspect is the shell of each drum - the cylindrical wood body. Shells should be completely free from cracks, and should have minimal to no scratches or scuffs. They should be almost perfectly round, which can be observed via the gap between the shell and the rim. This gap should be consistent all the way around the shell. Also check the bearing edges ( this is where the drum head meets the actual drum), make sure there are no chips in the wood and the edge of the drum is smooth and round.
Damaged shells cannot be replaced for less than the cost of buying a new drum, so do not purchase a kit with cracked or warped shells.
Shells can sustain damage through improper handling (such as being dropped or banged against things) or subjection to extreme temperatures or humidity.

 A drum's hardware consists of 2 rims that hold the heads on, tension rods that secure the rims, and lugs that accept the tension rods. Make sure none of these parts are missing or bent. Use a drum key to tighten each tension rod, making sure that the threads are not stripped. Note that small parts like tension rods and washers can be replaced easily if necessary. Make sure there is NO rust on the parts.
 If the drum set is being sold with cymbals, check their condition as well. Look for cracks along each cymbal's edge, and check for chipping (called "key-holing") around the center hole. Dull, discolored cymbals can be cleaned easily, but cracked cymbals cannot be repaired. Note that inexpensive starter cymbal packs are not meant to last very long, and you can always  purchase new/used cymbals.

Necessary drum hardware includes the throne (seat), the bass drum pedal, and the cymbal stands. These parts, if included in the sale, should be free from dents. All moving parts should operate smoothly, and threaded assemblies should be checked for stripping. And again NO RUST.

 Playing the drum set will tell you what a close inspection cannot - whether or not the drums sound good. If you are new to drums, consider bringing an experienced drummer along to the purchase to play the drums for you. Obviously, if the heads are bad the drums will sound bad, so take that into consideration.
Once you find a set that is in good condition, you still need to avoid overpaying for it. Look up the model online and see what it sells for new. If the exact model is no longer produced, look at other sets produced by the manufacturer. Ideally, you want to purchase the used set at a significantly lower price than a new one, with additional deductions made if the kit is damaged. Ebay is a great resource for prices, many pawn shops use this. Also, is there anything extra thrown in? Like sticks, pedals, drum key, cases, etc.
It shouldn't be too difficult finding the right drum set for you, just remember there are a lot of great local drum shops around that sell used kits. Usually they will throw in some new heads and sticks to the new drummer. So get out there and start searching for that perfect kit!!