So here is the definitive guide of what you should and shouldn't do on eBay.
First and most important step is DON'T go straight to eBay to sell the item, do your research for price on eBay certainly, but you should always at least try selling your item locally through free sites such as gumtree or local for sale pages on Facebook ,most important point here though is don't be greedy, say the item would sell for £100 on eBay, minus your eBay fee's, your PayPal fee's and your postage costs, you would actually really only be getting around £75, this is a HUGE 25% of your sale gone, this is why eBay should be a "last option" you have to accept that if you want to sell it locally your audience is much smaller, so you have to entice them to choose you, over any other seller on eBay, best way to do this is to take a hit yourself when compared to eBay prices, so if the item sells on eBay for £100, sell it locally for £80-£85 you will still be making more money than if you sold the item on eBay, and buyers will be more likely to snap the item up
its a win win situation,PLUS you don't have the "are these people going to be scum bags and say they never received the item" or some other sort of scam that you have to contend with on eBay.
eBay is rife with scammers just waiting to catch people out who post without sending recorded delivery (which means you have to pay a lot more for postage) or even people returning faulty or broken swapped item's, or even worse in some cases posting phone books back to weigh the box, fact is that eBay/PayPal will 99% of the time side with
Secondly DON'T sell anything that costs quite a bit more to post than the item is worth, eBay now charge the final selling fee "ON POSTAGE" (WTH i know, its ridiculous and nothing more than another cash grab from eBay to takes what little profits could be made on ebay in the first place)
here is an example so you understand how this now works
say you sell an item on eBay and it sells for £5, and it costs £15 for postage, ebay take their 10% fee's from the whole total so
£20 - 10% = £2 fee's off the bat
then PayPal will charge your their fee's which is out of the total again, so another
£20 - 3.4% -20p= 88p
leaving you with £17.12
then your postage of £15 if this doesn't include your packaging costs for fuel costs for going to the post office, for selling a £5 item eBay and PayPal charged a whopping 57.59% in fee's
so as you can see there is much more to the "simple fee's" motto that eBay claim
at the end of the milking process you will end up with £2.12, if you had stuck the item on gumtree you could of just got your £5, possibly even more as obviously someone was happy to pay £20 to receive the item, no reason why someone locally wouldn't of happily paid £10-£15 and collected the item instead, so in this situation, your would be receiving 700% more than you would receive on eBay
eBay is a great place for price checking, say you have some old snes games, and you don't know if they are worth anything, simply pop onto eBay search for the game then select completed listing and see the ACTUAL value of the items you have, so many people just go on and search and judge by the "buy it now" listings, fact is 90% of buy it now prices are ridiculously high, look for how much the item goes for when it actually sell's not how much some random person feels like demanding, if you start expecting to get the buy it now prices you will be very sorely mistaken, or be waiting weeks or months for a very specific person who may of had one too many drinks that night to pop on and buy an over priced item from you.
Just be sensible, look for an item in very similar condition (i.e don't compare the price of a unboxed or tatty boxed item to a factory sealed snes game) then look for several values of what the item sold for there will always be the odd extremely high sale price (idk maybe someone money launders on eBay or 2 drunk people entered into a bidding war) or the odd really low price where someone just got lucky (or unlucky for the seller) just work out the average price and then you have your general value. from that point its up to you how you proceed but i do recommend looking into eBay alternatives if you want to get the best deal
Ebay has its place, but unfortunately that isn't with the average seller no more, unless your are a company who sells millions of items, or you have a very "niche" audience which would be hard to sell locally.
Good luck and happy selling :D