regardless of whether a bike has been stored or ridden, you have the same problem buying any used bike, you just have no idea what its been through. Some things to check (in addition to what others have said)
1.) forks, make sure the seals aren't leaking (push down really hard on the front end a bunch of times, see if there is any fluid leaking on the chrome part of the fork tube). After an extensive period of not being used, I wouldn't be surprised if you have to replace the seals. Not a big deal if you do the work yourself or have a friend that can help you. I recommend OEM seals, call it lesson learned with the aftermarket products, especially if its a honda. This will give you a good indication if the tubes are straight or not.
2.) rotors, either have the seller take you for a ride, or go for a ride yourself, use the brakes when in 4th or 5th gear, do you notice and pulsing? if so, its likely a warped rotor
3.) pads, make sure the pads have good feel in them, both front and back, possible that the pads won't slide nicely after not being used for so long, do the brakes feel sticky/won't move easily/don't retract easily?
4.) engine oil, change immediately before riding, take a look at the smoke coming from the tail pipe, does it smell normal. If its blue, thats bad. If it smells like fuel, its fixible (running rich). Hows the idle? does it sound rough, or about right?
5.) brake fluid, change regardless after 2-3 years
6.) tires, look closely for cracks, push your finger in on the tire, does it feel hard or is it still spongy an soft? I always replace the tires, regardless of condition (unless they are clearly brand new). Most tires use a 4 digit numerical code (XXYY) where XX is the week of the year they were made, and YY is the year, i.e. 2910 would be the 29th week of 2010. Check the date on the tires. I don't like using tires more than 2 years old.
7.) chain, is it rusted, does it look stretched? Look at the manufacturers website for spec on how long the chain should be and how much free play it should have. Does it look like it's been cleaned/lubricated properly? Bring a measuring tape.
8.) gears, make sure you go for a test ride/double up and go through the gears. Since its only a 250, ask the driver to, or you yourself pin it (safely as possible of course), going through each gear, this will reveal the most of anything is wrong. Just becareful if the bike has been sitting for a long time, it is in an unknown mechanical condition.
