The absolute cheapest 420Wp panels are €50-80 new. That would be a roughly 1.7m X 1.1m panel. So 35€ for the risk of buying scrap and them having been degraded would be a bad deal. If you can get the rest of the needed installation parts with it, it might be worth it or if those are new panels.

If you want to get started there are solar home kits online, which are rather easy to install, if you have some very basic diy skills. The electronics is basically plugin in some cables and the difficult part would be actually setting up the panels. On a flat roof that might just be screwing on some metal stands and drilling a hole to run the cables through.