Good places to check for used or lower-cost studio gear include:

• Used camera dealers such as KEH and Adorama Used. They may not always be the cheapest, but they usually grade condition reliably.
• Refurbished gear from retailers like Adorama can be a useful price benchmark—if refurbished costs a certain amount, you generally wouldn’t want to pay more than that for used.
• Rental houses selling retired gear can be worth watching. Their equipment may be heavily used, but it is often maintained and serviced.
• Community buy/sell forums such as Fred Miranda are often good sources for studio accessories like light stands from established sellers.
• General marketplaces like eBay and Craigslist are also options, but condition and seller reliability vary more.

For backdrops specifically, availability may be more limited at used dealers than for tripods or stands. When buying used support gear, check load capacity, locking mechanisms, bent sections, and signs of instability.