This guide is just from experience, and I don't wish to name any names or point any fingers.
When you are trying to decide whether to list an item as available for selling overseas or just for the UK, there are some really obvious pitfalls. The idea that someone would love to buy your expensive product but can you just wait for the Western Union money transfer etc is obviously a scam. Similarly, lots of foreign buyers will ask you for an "off e-bay" price or a price to end the auction early. If they are willing to pay £100 for an item to end early, then they are willing to pay £100 for the item full stop. The only thing you lose by running the auction to it's scheduled end date is the chance that someone else will pay £101 and you will be better off. Don't deviate from your original plan.
When someone buys from abroad, it all works just fine unless there is a problem. Sending something back from the Phillipines is a lot more expensive than sending it back from London. Postage costs both ways are higher, and if you need to offer a refund, do you offer a refund of the postage costs as well?
You may disagree about the condition of an item with your buyer. No matter how simple or genuine your disagreement, it is impossible to resolve when the cost of an independent inspection is £25 plus a couple of thousand pounds in air fares and hotels. Do you just give up and send the money back? Do you argue and risk being seen as dishonest? I don't have an answer, I'm just pointing out the problem.
You may have a reason to make a full refund. What happens if you decide to get the goods back and agree to a full refund? The buyer then suggests that on top of your original £100 refund, you must refund any import or excise duties he has paid, which might come to another £100 again. You are then in the position where you will have lost money only to be left with the original item which is then older that it was when you first sold it.
In the event of problems, paypal will not mess about, they will simply take back the funds from your account. This leaves you unable to pay any suppliers, and in the meantime your goods are still two continents away. So to sum up I would suggest that you only sell items which are brand new, and you only accept payment via cheques through the post. Make it clear in the listing that any returns will not be accepted, or if they are then you will not be liable for additional costs such as duty or excise. Don't send the goods until funds are cleared into your bank from a cheque, and when you do send them make sure all goods are properly insured. Don't accept paypal, don't accept western union and don't agree to a special deal for the Nigerian Pastor who is only buying your expensive electronic goods to help his poor flock in the back of beyond.
Good luck, safe ebaying.