Neopia has three ways in which you can sell your items. You can sell items for 1-99,999 NP in your shop. You can trade items in the Trading Post for any price. You can put your items up for auction. Which is best? Well it all depends on what the item is, and what you know about it.
For any beginning player, I absolutely recommend against using the auctions. Usually, you get ripped off. The auction is so packed with junk right now, that your item will unlikely get the coverage it needs to get the price it deserves. (At any given time, there are upwards of 200 items pet minute being posted to the auctions.)
Rather, use the shop wizard to get an appreciation of what your item is really worth. Do several searches, the reason being that the shop wizard only searches some of the marketplace at a time. Once you have a feel for what your item is worth, put it in your own shop for a few NP less than the lowest price you saw. If doesnt sell in a day or two, search the shop wizard again, and re-price your item.
When using the Shop Wizard, make sure you have the wizard set to identical to my phrase, and that he repeats the phrase when you make the search. If he doesnt, the item doesnt exist, or you have mis-spelled it. If, by chance, you dont see your item in the shop wizard, then congratulations. You may have something very valuable on your hands. Dont put it in your shop for any price, rather, visit the trading post, and search for your item. See what people are asking for it. When youve gotten an idea of what its worth, post your item on the Trading Post. Make sure you are specific in what you ask for in return. You should only ask for pure NP, unless you know exactly the value of an item you would prefer. For instance, say you have an item and everyone else on the Trading Post is asking for 150,000 or more, then you might post it and ask for, 148,000 Pure = Auto, meaning that if someone offers you 148,000 NP, you will sell him your item. Be sure to be cheapest, or you wont get offers. But beware people trying to deflate the price of an item on the Trading Post- if 40 people are asking for 200,000 NP or more for an item, and then one or two people are asking for 120,000 NP, they are trying to trick you into selling your item for less than 120,000 NP. They have no intention of ever selling the item for 120,000 NP, they just want you to sell yours for cheaper, so they can buy it and resell it at the higher price. Its a very common practice on the Trading Post, so keep your eyes peeled. If a price looks too good to be true (in comparison to others) it probably is. In this case, you would post your item at a slightly lower price than 200,000 NP. Not lower than the 120,000 the scam-artist is asking for. Also watch out for people making bogus offers on your items. Dont accept any offer before youve carefully looked at it. If its your first time using the Trading Post, youll notice that even if you asked for Pure NP, the offer will include both the NP, and some worthless item. This is because you cant actually sell an item on the Trading Post, but you can trade it for an item + any amount of Neopoints.
If your item is sellable, but you cant find one for sale on either the Trading Post, or thru the Shop Wizard, then you may have hit the jackpot. But since you dont know what its worth, be very careful. For now, put it in your safety deposit box, and consult people using the Neoboards until someone can tell you what it is, and what its worth. When you have an idea of the value, you have two options- using the Trading Post, and letting everyone know you have the item via the Neoboards, so they can make you offers, or by using the Auctions. If you decide to use the Auctions, you need to be absolutely sure that your auction is long lasting (1 week), and that it is very well publicized. Otherwise, you may lose your priceless item and get a very low offer on it, which you are forced to sell it for. I would personally use the Trading Post, where you have control over whether or not you sell your item.
Lastly, dont send thank you notes. They are annoying and time consuming to read and delete. Unless something special happened during the deal, there is no need for one. Good luck, and always get the best price you can. – DragonDoodies