Pick the right version
Consoles come with or without a disc drive, with different storage sizes and across generations. Check that the exact version supports the games and accessories you want to use. The same manufacturer name doesn't guarantee compatibility with older controllers.
Digital games on the seller's account are not automatically part of the console being sold. Work out the value from what is actually handed over with the device. Check separately that the console lets you use the online services you need.
Check that it works
Testing a console means more than switching it on: video output, connections, the disc drive if there is one and how it runs under load all matter. Excessive noise or shutting down may need further diagnosis. Look for a specific list of tested functions in the description.
For a controller, check the buttons, analogue sticks, charging and connection stability. Stick drift in games can mean extra repair costs. Check that the power and video cables you need are included.
Compare the costs
Compare consoles with the same contents. An offer with one controller and less storage isn't directly comparable to a full bundle. Before you bid, leave room in your budget for the accessories you need and consider how far the console has been tested.