Well I've sold on eBay for years - all sorts of stuff.
Things to take into consideration...
eBay fees can be 10% of total selling price (which includes the postage cost). However, as a long standing seller, I get special offers every now and again with max. selling fees of £3 or £1.
eBay also sides with the buyer every time, so be aware, anything of value I always post tracked and signed for with the correct insurance, especially when sending abroad.
PayPal costs can vary - selling to a UK buyer, PayPal charge 3.5% +0.20p, for Europe and elsewhere it's a flat rate 4%.
In the event of a dispute, PayPal side with the buyer unless you can prove otherwise and will block funds on your account.
Also, always wait until the funds have cleared before sending - some people use a method (eCheque) that can take a while before funds are cleared.
When selling something, do your research and be realistic with your pricing.
It's sometimes better to sell on forums as you can offer the item at a better price. However, not everyone is trustworthy so apply some of the things mentioned above.