The key to retired items is patience. Timing also helps... but you have to be prepared to wait it out.
This method of making NP is simply mass buying items that are no longer produced (retired) and storing them away until they are more valuable. Items that are used, lost, burnt, stolen, frozen (with the account), and discarded reduce the amount of these items in Neopia. Demand will slowly raise the price of these items as they diminish in number.
I took it upon myself to record the price changes so I could determine whether I was better off leaving my NP in the bank to gain interest... Over the years this database has grown. It started as a singular effort and then grew to what you see now. As I have been monitoring these items for several years, I feel that I have some knowledge on what items are likely to produce the most profit. I have learnt this the hard way at times.
First off... the best retired items to buy are those that are one use items, and avatar items. Coupled together (especially when the avatar is gained by using the item - it disappearing) and you'll be able to make millions off a handful of items. Thou you will have to have those items already (requires planning, foresight, and luck) or be very quick and monitor the news and chat boards constantly.
Otherwise you can still make roughly 2x to 10x of your investment a year. Although this wont be your total investment. Some items will increase more than others, and you may only make this sort of profit from a selection of your items.
Things to note:
There are so many advent calendar items in Neopia that they rise in price slowly. Although they are a cheap investment, most starting at roughly 50 - 200 NP each. You can make up to about 2x your investment each year like this (better than the 11% that the bank will give you)!
Books, foods, stamps, petpets, and items with novelty or appearance value will go up in price relatively fast. Some will double in a month. Most will go up in price up to roughly 10x there initial value within days of becoming retired.
When it comes time to sell:
Some people make a good living off really high priced items (they have the funds, shops, and experience to do so). You may find that someone will buy all af your (eg) golden carrots at 50k (when you paid 10k) and be happy about it. This may leave you wondering whether you have sold for less than they are worth.
Other items will sit in your shop (trades) for months, at far less than others are selling for, with no one caring.
I think this comes down to how popular the items are. Other fortune seekers will be happy to take your stock off your hands (I've done this for several people - who contact me offering a bargain price for a large amount of items).
If your set of items have increased in value but just aren't selling, offer them on the trades for items that will sell. My petpet shop is occasionally stocked from palming off HTS (hard to sell) items through the trades. Most people will check the price of your item against the wizard and the trades, see the price and accept your offer. Just remember that you paid very little for your stash and can afford to offer a little extra for a quick sale.
The best way to handle this is to have a range of retired items in your shop (1 or 2 of each item). Price then at over 1000 (so it will show up in your history), and between a few thousand and 2x their current value. As time passes the value will increase and you will make a sale. You check your sales history, replace the sold items, and increase the price. Over time you will make sales that can fuel further purchases, and you will see the benefit of your investment as a steady profit.
Otherwise you can simply hoard items away and wait (years if needs be). Record the prices you paid, and have a sell point (much like the stock market). Once the items reach this price, you sell.