Like fietsbob said, "they are aftermarket add -ons.. not in the carton, the bike ships in, from QBP". Depending on brand of racks and fenders, it could cost as much as $150 or more to outfit a new bike.

If the bike is a stock bike, you pretty much know the value of the bike with the standard components. If this is the case $850 might be a reasonable price. If the bike was built up by the seller, you should look at the components and make sure they are at least as good or better than the stock bike.

Also, look at the condition and factor that into the decision making process. If you know bikes, check out the wheels, brakes, etc. If not, ask a knowledgeable friend or acquaintance to look it over for you.