Buying a bike or a kayak with a few miles on it, or a weight-training circuit that’s been sitting in someone’s basement collecting dust can save you hundreds of dollars. Website FitnessOutlet.com sells refurbished workout equipment like stationary bikes and elliptical machines. At PlayItAgainSports.com, you can search a nationwide network of stores where you can buy everything from shin guards to surfboards — especially handy if your kids are outgrowing their equipment as quickly as their hobbies are outgrowing your budget. The one caveat here is helmets: You need to buy those new because there’s no way to tell if the structural integrity has been compromised.