Here is what I know based on my former experience as a Canadian Customs officer and my observations on how the US Customs system works (relatively similarly)
If you are importing a $2,500.00 item that you purchased used, it doesn't matter if it is used or new, it still may be subject to duties, and there still are regulations applicable to the importation.
If you are dealing with the return of an item that was sent out of the country for repair, the situation is different but while most likely there is a special tariff item for return of repaired merchandise that renders the importation duty free, you still may be required to declare the goods and claim that particular exemption. There are some additional complexities, but I'm sure they don't apply to cameras.
When you get a courier to bring items from outside your country to you, whether you purchased them outside the country or had them repaired outside the country, you are naming the courier as your agent for the purposes of the importation. If you had, instead, brought the camera into the country yourself by showing up at the border with it, you could very well be required to fill out that form and give your Social Security number. If you are required to provide it with a personal importation, your agent (the courier) will have the same responsibilities.
The volume of cross border shipments means that requirements like this aren't always enforced. There are also thresholds in place that result in the requirements not applying to low value personal importations. If, however, the customs officials have suspicions that the importations are being done for a commercial purpose - such as a commercial photographer importing equipment for their business - then the rules and requirements are much more rigorously enforced.