I'm just an amautuer seller / buyer but have some tips I think work. I'll keep them short and sweet
1/ Always keep your postage costs down. There are thousands of people out there, including me, who hate the sellers who load their postage. You know the ones who sell a 99p CD and charge £10 postage. Real cost should be about £1 which would include insurance.
2/ Always offer insurance. Personaly I try to avoid sellers who don't offer it.
3/ Check out Royal mail web site ; royalmail.co.uk , it has all the postage costs on there, including shipping abroad. You can then quote actual costs instead of guessing
4/ If the value of the item you are selling is under £30, post 1st class and obtain a receipt of posting from the post oiffice. It is then insured for free. The number of people I see offering 1st class post and show insurance 'not offered' is crazy. It gives the buyer peace of mind.
5/ If the value is more then post Parcel Post and for 75p you can insure it for up to £100; £1.75 for up to £250; £3 for up to £500.
6/ Don't give the buyer an option on insurance. It sounds nice to offer it but is a nightmare if something is lost or damged in the post. The buyer won't believe you posted it or will think it was already damaged. If you keep the postage costs real you can offer insured postage cheaper than thos who load there postage
7/ Keep packing costs down by re-using the packaging from items you buy.
8/ Pack well. Pack as if the item is going to be thrown about. Use loads of tape.
8/ Check the weight of what you are selling. Parcel Post will take up to 20kg parcels ! I often see sellers saying they won't post because it weighs too much but the item is obviously under 20KG, so they haven't even bothered to check if they can post it. No postage means lost sales.
9/ By all means check out other couriers but for the amautuer I think the post office offers the best value and it's easy.
10/ Always add a picture to your listing, if you can. I usualy don't bother looking at auctions without one.
11/ Get yourself a Paypal account as quickly as possible. Yes it costs a few pence each transaction, but it's quicker, safer and again I avoid auctions that dan't take Paypal, so agaiun lost sales.
12/ Have only ONE credit card set up on Paypal and don't use that card for anything else. All my fees and purchases go through it so I can keep track of the money.
Hope these help