DSJ - a few initial hints:
1. Take good clear pictures - at least one pic is always free, and eBay won't accept listings without pictures these days.
2. Always describe your item in detail. If something's a bit wrong (scratch, mark, etc) say so - buyers can't then say you didn't tell them.
3. Weigh your items and potential packaging meticulously so you allow enough for P&P to charge to the customer (you can get a good leaflet from the PO or the Royal Mail site - you have to measure parcels these days as well as weigh them, to get them in the right category.
4. You can charge a little for packaging materials as well as postage, and for petrol/parking - a few extra pence on top usually covers this.
5. If you overcharge on P&P in error, you can always refund an amount to the buyer if you want to keep them happy. I do this if I have wildly miscalculated, or if a package goes (e.g.) as small rather than medium package rate.
6. Above all, make that description true and honest.