As accountantpete says, think about the item your're selling. If it has value, but is too heavy to post, have it as a local pickup only, BUT make sure it doesnt go too cheap. You can do that by making your starting price the minimum you're happy to let it go for.
Remember the fees...its somewhere in the region of 10%, then paypal fees as well. Between them, they take a fair percentage.
Be as accurate as possible with your description, especially if you're posting it. There's nothing worse than an unhappy buyer, and you'll bear the cost of putting it right. Ive fallen foul of that recently...one missed important piece of info, buyers unhappy, refund and bear the cost of the buyer returning it to you. For me, its a one off, so you just swallow it.
A good photos helps, and make sure your auction doesnt end at a funny time...you'll get fewer bids.
For smaller or lighter items, use the Royal Mail portal online to calculate postage. Just make sure you select the correct package type (letters or small packets), select the weight and it'll tell you how much it should be.
Be prepared before you list anything, there's nothing worse than selling an item...and not knowing how you're going to send it or pack it.