1) Be kind to the buyer or seller, if there's an issue, you want to resolve it. If the item that you are selling doesn't arrive, give the seller a full refund and try and deal with the rest via Royal Mail themselves. You just want to be perfect. Your feedback is more important than a £2 blanket. Seriously, don't underestimate you feedback. Be kind, and people will be kind to you.
2) Send your item out quickly. Quickly is very good, because people will be happy. Making people happy is your goal. If the buyer/seller is happy, you should be happy. Your feedback score is very important, remember this. Often I hear people thinking of slightly scamming their buyers. This is a 100% NO! Scamming is horrific. Any scammers should receive 100 negative pieces of feedback at the very least. Scamming is a HUGE NO!
3) Be polite. Being polite is just another form of kindness, and will help you all the way. Being polite makes the buyer/seller happy, which should make you happy. A happy buyer/seller is good.
My most important tip is: KEEP THE BUYER/SELLER HAPPY!
It's so important, I always end up asking myself, who do people favour themselves instead of their buyer/seller.