What is too many clicks for a canon 7d if looking to buy it used?

I wouldn't worry about the 18,000 clicks, if the rest of the camera looks good, and functions properly.

An EX condition 7D at KEH.com is $820 and EX+ is $850.

http://www.keh.com/camera/Canon-Digital-Camera-Bodies/1/sku-DC029991118450?r=FE

Here are the used 7D prices at Adorama they range from $750-$1145

http://www.adorama.com/searchsite/default.aspx?searchinfo=used+7d

and ebay search of completed listings was similar.