It all depends. At 8 years old, you're almost better off if there is NO suspension. I've seen 8 year old bikes that have been used 2 or 3 times and then collected dust in a garage.
The first thing to determine is if the frame size is appropriate. Bike shop bikes come in different frame sizes. Take it for a test ride. If your "boyz" are toching the top-tube chances are it's too big for you. If you feel like your knees are hitting your elbows it's too small.
Other than the basics. Do all the cables work? Do the brakes work well? Are the tires dry-rotted? Does the bike shift through all the gears?....etc.
Look for cracks at the welds of the headtube (where the forks meet the frame), and at the bottom bracket (where the cranks go through the frame).
Are the wheels straight?
Is there a lot of rust?
SO MUCH to consider. Also, remember that bike BRAND NEW was probably only $300.00, so unless it's perfect, I wouldn't pay more than $60.00!
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger