This is just a general guide to why things go missing in the mail, most of it blindingly obvious, but having been a postman for almost 20 years, I have seen most of this first hand.
So, firstly. When something goes astray in the post, your first instinct, and admittedly mine too, is to blame Royal Mail. After all, you KNOW you put the correct address on the item. Just sometimes though it isn't Royal Mail's fault.
Yes, mail does get mis-delivered, far too often, but in my experience, speaking as an ex-employee of Royal Mail ( I have no love of RM by the way...the buggers fired me, I just hope what I know is useful to you), the vast majority of stuff that goes missing isn't the fault of the postman.
Every single day, every delivery postman has letters to "kill off" and they are the bane of a delivery postie's life. Most of these will be "Gone Away". But you will always have some items each week you just can't find a home for. This may be because on attempting delivery, the occupier has handed it back to them saying it is "not known at this address". Or the occupier puts it back into the system with "not known" written on it. This will be returned to the delivery postman to endorse. A good and conscientious postman will try and find a home for it. If he is a regular on that round, he may know the surname. Or he may look the surname up in a phone book. Sometime the Street or Road just does not exist in that town.
Some postie's really don't care, after all they aren't paid well and you can't really blame them for not going that extra bit. It is also against the rules to deliver something without a correct address!!!! So every time a postman DOES attempt a delivery with a vague or incorrect address, they are making the effort on your behalf at risk to themselves from over zealous managers.
You may even be lucky enough to see something like this:
Incorrect addressing of mail is such a problem, that there are special hand-stamps to flag up to the customer the error in the address. Where I worked, Scarborough, it was quite common to end up with mail for Torquay. Why? because someone had not taken care in writing the address. Scarborough's postcode is YO12. Torquay is TQ12. If someone had scrawled an address so the T looked like a Y and the Q looked like a O, then someone else had copied it into a computer somewhere as YO12, then forever more, until the data was corrected, the mail would be read by sorting machines as YO12. See how easy it can happen? We also ended up with a lot of mail for Whitby at YO12, because Whitby's postcode is mainly YO21. A simple mistake, but one that can delay your item or end up with it lost completely.
In my experience, the majority of mail that has to be returned to sender is because of incorrect addressing by the sender. One simple thing will help you here. Short of putting the correct address on in the first place.
USE A RETURN ADDRESS!!!!! On the outside of the item. Royal Mail prefers this to be on the reverse/back if possible, as 2 addresses on the front can lead to your item being re-delivered to you.
Why use a return address on the outside? Simple. The postie who has to endorse the item with a reason for non-delivery also has to make a mark through the address (normally with a blue crayon or marker pen) to "kill" the item. Then he will ring a return address and straight away, that item will be on its way back to you. You could, at best, have an item returned to you within 2 days of posting it. It will save you time and trouble chasing it up and you will be able to tell your buyer why it has been delayed. It may take longer than this, but it is normally within a couple of days of it being "killed off" .
On the contrary, if you do NOT put a Return Address on an item. A different thing happens to it. After "killing the item", the mail then has nowhere to go. So RM sends all undeliverable mail to Northern Ireland where the RLB is ( Return Letter Branch) It goes to N.Ireland as they have the x-ray machines to scan everything for threats. In case anything inside may pose a threat to the staff opening the mail.
Your item will then be opened and if an address is found, it will be returned to you free of charge. With the varying amounts of mail that ends up at the RLB, your item may be delayed 3 to 4 weeks. Sometimes longer.
In this time, you will have been in a debate with the buyer who hasn't received the item. You have paid postage. And you have been blaming RM for losing it... or worse... your postman for stealing it. You may even have had a negative feedback from the buyer if you didn't get a proof of postage.
How much simpler would it have been to have put a full RTS address on the rear of the letter??? ( while a short version works, say 48/SW1 1AA, it is not ideal. A street or road name is much much better. So 48 smith st, SW1 1AA is perfect.)
Another very simple reason for the item not being delivered, or rather the recipient not receiving it is, they haven't updated their address after moving house that you get to see on the invoice. They may not have logged on to ebay in months, but one day they do, make a snap purchase, pay, then sit back and wait for it to arrive. Except it has gone to the old address. They fume at you, you swear blind you sent it and no-one is a happy bunny.
With a RTS address on the outside of the item, you will be able to tell them much quicker, that it has been returned to you from their old address.
I'll add more to this later.... any feedback is welcomed, but not if it is just a moan at Royal Mail... I've heard that all before and there isn't anything i can do about it :)
(more added 18-1-07)