and from experience...Tips on buying used bikes:

Inspect the bikes thoroughly, or have someone that knows bikes go with you (and not know of them, but actual experience working on them for a living). Preferably one familiar with that particular bike/make/etc.

HAVE HIM RIDE IT...not for 5 minutes, have him go on at least a 10-15min ride if they will let him. It is easy to get a bike to idle okay for a few minutes, but after the bike is really warmed up it will reveal a great deal if there are problems with it (engine/tranny/suspension/etc).

If they really want to sell it, they will let you (but please make sure it is an EXPERIENCED rider, you don't want to have to pay for a totalled bike if they are loopy).

I can not stress this next point enough...CHECK THE #'S yourself online. Make sure that the frame and engine match each other and the title. If they don't match each other it is not terrible, just lets you know that it has been swapped out for some reason.

If they are different from each other, make sure to check the #'s online individually. I have a friend whom lost a bike to the police impound because the engine was stolen, and just not reporting it (even not knowingly) is accepting stolen merchandise because the police will say you should have checked. A real good deal gets ten times worse when shit happens...and you know what they say...

If they match and they match the title, your all good.

IF THE FRAME VIN# DOES NOT MATCH THE TITLE DO NOT BUY IT. It will not be worth the trouble. Do not buy anything that the person says, "it has a title, I just don't have it...all you have to do is...etc, etc, etc. Sometimes a deal can be made for a really good price, but it is rare that it doesn't have ANY snag or it wouldn't be that cheap, I don't care what they tell you...be smart, don't be a retard.

It is easy to get caught up in someones bullshit story while looking at something you really want...and sometimes the pretttttty paint and ssshhhiiinnny stuff makes you go ooooooh, but use reason. You don't know how many friends I have that in the past squiddage days bought the 1st bike they looked at...bad idea, no matter how much he tells you the deal won't be there in a little while, more often than not, it will be.

As for the mechanics, leave it to someone that knows better as stated above...but for a general check:

ASK THEM ABOUT ANY PROBLEMS
Check the fluids obviously
See if there are any areas that are leaking fluids, have large deposits, etc.

Look for broken pieces, clips, tape (holding stuff together, like wonder duct tape), pieces of wire/zipties/string even are usually the signs of a lost bolt/nut/screw (no biggie), but the tab/mounts could be broken off and not easily repaired (pain in the ass).

Listen for tapping, knocking, irregular noises, etc...if you here them find out what they are if you can.

See if it overheats, stalls, smokes, all the obvious things...also keep in mind most bikes do sound and run rougher when cold, but if it does have it checked first.

Look for any scrapes, grinds, etc...on metal or plastic. Look for multiple replaced parts (sign of a crash). Not that a crashed bike is no good, it is just better to be aware of it as you will scrutinize the bike more if you think it has been wrecked.

Just ask lots of questions and look for the "to good to be true" type deals and think clearly about your purchase, do your own research...you will learn a lot.