When buying used or parts phones, there are some major things to look out for.

Know what you want to buy first, there are lots of different phones in many various conditions for sale here on ebay.  They range from brand new to completely useless.

For the sake of time Im going to stick with Motorola's line of Iden technology phones.  These phones range from the original Iden phones to the newest camera phone mp3 players on the market.  But if you are going for used phones you have to know a few things before you buy the handset.

1.  The IMEI number of the handset you are purchasing.  You should always ask the seller for this number prior to your purchase.  If the seller is hesitant to give you the number, be cautious, getting a phone with an imei that belongs to a lost or stolen phone can give you problems.  There are 2 ways to check the imei number on a phone;  1 place it is listed is on the back of the phone, on a sticker under the battery, now, this number can be easily changed, or if the phone's housing was changed during a  rebuild or for other reasons.  You may also want to check the imei that is actually stored in the phone.  The only true way to get the imei is to go into the phone, open the menu and go to "my info" then while in my info, hit the menu button again and scroll to "imei/sim/id/sn"  This will give you the imei of the handset as seen by your carrier when the phone is first turned on.  If you suspect an imei number, or just want to check on one before you buy a phone, you can call your carrier and inquire about that specific number, i believe that you can also go to a local retailer of your phone and have it checked that way.

If the Imei number has been reported stolen or lost then you may not be able to use the phone, and you definately won't be able to activate new service on it.

Getting a phone with an imei number that does not match the sticker on the back does not necessarily mean that you got a bad or stolen phone.  This may just mean that you got a phone that was rebuilt by someone who fixes phones and was careless with the original/matching sticker for the phone.  It however could also mean that you got a phone that was stolen and a non-stolen sticker was placed on the outside to throw off an honest buyer.

2. Water Damage.  Phones that are purchased used or for parts should always be checked for water damage/liquid damage.  This is easy for some and harder for others.  Some phones will continue to work if they have been exposed to water or other liquids, but cannot be serviced if you run into problems down the road.  To check for water damage you have to take the phone apart.  While this is not true in all cases, it has to be done on almost all phones, only recently have phones been coming out that you can tell without taking it apart if there is any water damage. 

Either way, the phone can be disassembeled by anyone with some competant knowledge of how to take things apart, provided they have the right tools.  Such tools can be purchased here on ebay.  What you are looking for when the phone is take apart are 3 things:

The first and most obvious thing is the water indicators, all phones made for nextel within the past 2 years have these indicators, they are small white circles that look like hole punchouts froma  3 hole punch.  They are glued or stuck in 2 places on the mainboard of the phone.  If they are white the phone most likely doesn't have any water damage,  if they are pink, the phone was exposed to some level of liquid, possibly only for a mere second, or a very little amount of water over along time (outdoor use, use in very foggy weather, etc)  if they are pink the phone may be ok, but most repair facilities will take that as enough to call it water damaged.  If the stickers are RED, then the phone was exposed to alot of water, most likely fully submerged. 

the second thing you want to check for is discoloration of the metal covers on the inside of the phone, they should be usually a bright shiny copper color with minimal to no discoloration.  If there is discoloration the phone may have been subjected to a cleaning in alcohol and then baked, cleaned again, etc, etc.  This process is used by many repairers in order to break up the corrosion that occurs when a phone gets wet.  This doesn't mean that you phone is no good, but if a repair facility notices that the covers are discolored or unusually colored, they may decline future service on the phone.

The third thing you want to look for is corrosion.  Corrosion is what ultimately makes a phone fail after water damage.  Corrosion is developed almost immediately if a phone is on when it hits water, this causes short circuits and "hot spots", causeing the phone to work erradically or not at all.  the corrosion is sometimes hard to spot, and even if you do fine some, it is not necessarily from a water incident.  so try and use your best judgement.

Be aware that people have and do sell the water indicator pads on ebay and elsewhere in order to make people think that a phone doesn't have water damage when it really may.

Just remember, if the phone is fully functional and doesn't get hot or look funny on the screen(s), then you may be ok, just be aware that some people try to pass off bad phones as good ones.

The next thing to look for when purchasing a phone is physical damage.  Try to ask your seller as much as you can if you see a cracked screen or broken antenna, sometimes it isn't just about what is listed.  As with a car, there can be underlying problems that may render the phone useless or very expensinve to fix.  Usually when an antenna breaks, it is just the antenna that broke, but under some circumstances it can cause damage to the mainboard of the phone, sometimes this damage is easily repaired, sometimes it is not.  I personally purchased a phone that was "slightly used with broken antenna"  and when I got it I just threw a new antenna in it.  The phone still would not get a signal, so I took it apart to check it out.  When I got it apart I noticed that the whole corner of the circuit board where the antenna contact is was broken clean off, and not only that, it was MISSING, meaning that the seller knew about the problem, but wasn't honest about it.  after many an e-,mails and a claim through paypal, I finally got the story that the phone was RUN OVER!!  it was strange though, everything else on the phone functioned perfectly, it just couldn't get a signal.  Either way, always look at the listing carefully and remember that "just a fender bender" could mean "totalled"

I can't really think of anything else to add, but please keep these things in mind if you are purchasing a phone, Always try to get as much information about it as you can, and if you really feel confused, then buy a new one.