Bows depreciate in value like a car. As soon as you drive it off the lot, it's used. ArcheryTalk is a buyer's paradise...some guys on there will buy 20 bows a year, every year, then run maybe 100-200 arrows through them and sell them. Basically what they done was settle the string/cables for you and gave you a huge discount. Like Dixie said...in a lot of cases, you can get parts by paying for them, and they really aren't that bad of cost if something does go wrong.
With that said...I enjoy having local dealers, and if I really like the dealer, I will just go ahead and buy new from them vs used. Yes I pay more for it than a used bow, but when I like them, I know that it helps support them and a lot of problems can be fixed at the dealer's shop too. But I must add, there are very few that I choose to support, and if it happens to be a bow model that I want that they don't carry...used is where I go.