Sound advice above. I would add that if you are selling higher value items (e.g. locomotives) that you use Royal Mail Signed For and for lower value items ordinary post is fine but, as fozzy280472 says, never post anything without going to a Post Office and obtaining a Proof Of Posting (no additional charge and gives you some insurance cover).
Unless you are in a hurry why not list a few low value items and let the whole process run through from listing to seller receipt and, hopefully, positive feedback? This will allow you to familiarise yourself with things and also get your feedback rating up which should provide some reassurance for future potential purchasers. I did this years ago, starting with a couple of Airfix kits sold for a few quid. Since then I have bought and sold thousands of items and problem transactions are still in single figures.