Easist way to ensure safety of buyer and seller...
1. Make sure you give enough pictures that depicts every little blemish. Obviously you'll want to see the cash in your account first before you ship and it's only fair that they know what they're going to get in advance for their hard earned cash. Put a piece of paper with your ID and the buyer's ID in every picture so that the buyer knows that you have what you're going to sell in your possession. You both need to be happy and comfortable with the deal before you proceed!
2. Decide how you're going to get paid...
2a. You could take a purchase payment from PayPal and get hit with the fees (possible opportunity to negotiate and split) which gives the buyer some protection should something goes wrong in shipping and it gets lost. There are some implications with this as you usually have to refer to an advert for PayPal to refer to and this may be restricted to fleabay. Speak to PayPal (both buyer and seller) and enquire how it works would be the best thing and then discuss with each other to ensure you're both on the same page. Payment clears almost instantly.
2b. Take a wire payment. Obviously you'll need to hand out all your personal details (Name, Address, Telephone, email, Bank name, Branch, Branch / sort code, account number, Swift / BIC code and IBAN for Europe sellers) so that not only can you get paid but also so that the buyer is comfortable that they can verify that you are who you say you are and where you live. Beware of people that may just use this as an excuse to get your details! Whenever I pay via wire I always ask for this information and make checks using the white pages, etc. Payment clears in around 3 days.
2c. Combination of 2a and 2b. Take a $100 deposit, for example, as a personal payment (minimal fees) before you give your details out and then take the rest by wire under 2b. Best security for minimal fees. Full payment clears in around 3 days.
2d. Customs. This isn't a payment to the seller, although there is a way I believe for the seller to pay in advance on behalf of the consignee. I don't know how to do this so don't ask
A useful link for customs and tax calculations for buyers in Europe is:
http://www.dutycalculator.com/
3. Shipping...
3a. Labelling - Mark on the package and export form where applicable 'Electric Guitar - Used'. This tells everyone exactly what it is and should speed up the customs part of the shipping, preventing any unnecessary delays. You may need a Harmonisation Code (HS Code) which should be 9207.90.00 for an electric guitar.
3b. You can ship by USPS for around $100 - $150. They usually only offer around $650 max insurance so there would be a need for additional insurance from the buyer's side. If the buyer is a UK buyer, you (
fractionallz only please) can send me a message of the buyer's ID at the buyer's request and I can put them in touch with a very good insurance company. The onus is on the shipper to declare the value of whatever they're shipping. It may not be an issue but I feel that it must be mentioned officially.
Disclaimer: Neither I nor Jemsite are condoning inaccurate declarations / potential illegal activity in respect to Customs and Taxes - or anything else for that matter 
3c. Ship via UPS, TNT or whoever and insure for the full amount. This will cost significantly more and you obviously will have no choice but to declare the full value.
4. Pass on the consignment number and depending on who you've shipped it with, it could take up to 2 weeks. The fastest I've experienced shipping from USA to UK was 9 days via USPS, but that included me going to the depot on a Saturday morning to collect the consignment and paying the customs fees there and then. Be aware that shipping time usually does not include the time it takes to go through customs. The shipping time quoted on these services is usually in business days, Monday to Friday!
