Thorough research is a crucial step in understanding what issues users may experience with their device. Remember, you’re helping real people keep as much stuff out of landfills as possible!
Here are some research strategies for an example device (Canon PowerShot SD900):
Google search "Canon EOS Rebel T5i won't turn on,” "problems,” "repair,” etc. If you find repair-oriented or parts sites, the parts that they sell will clue you in to what people need to replace.
Use enthusiast sites like this one—they will pop up in searches. Browse through them to get a feel for common problems.
Feel free to research similar devices on iFixit.com for reference.
Beware of Fixya. Fixya has some information, but confusingly mixes in other information from completely unrelated devices. Use information from this site with caution.
Some devices have little to no repair information about them (either because of manufacturer frugality, or simply because no one has bothered to try and fix that device yet). Don't let this hinder you! Rather, use common sense and think of some problems that could possibly arise, i.e. not turning on, sticking keys, or black screens. You can also research problems with other, similar devices; odds are, some of them will apply to your device as well.