Neopets Economy has always been a luxury economy. There is no permanent adverse effects for not feeding your pet, or not playing with it, or having it be sick for a month. You don't need to get anything, the very lest you need is given to you freely and with just one click. There is no urgency to get anything. Other than the time you're using for training and the threat that some items might be retired (or already retired/special items will rise in price/disappear) you can take your time to get anything. Which means people are only pushed to buy when they believe the price is right, and that said price won't be lower in a future date.
That's what's happening right now. Savvy people have already realized than pure neopoints are worth more than items right now, they're as likely to invest in items because due to several factors (New RS system, Charity Corner, smaller active player base, etc...), thus the only people buying items are: People looking for items for their collections, which, if well aware of the current situation, as most old players are, will ask for discounts, and depending of the item, the players will grant it to them, because, who else is gonna buy said items? Toys, Usukis, MPs, Rare BD weapons with bad stats, even Books are deflating as more and more people are waiting for them to go even lower. The only other people who will but anything will be reselles, trying to buy for an even lower price, sometimes as low as 40-50%, hoping they can sell for 70-80%.
A third bad thing is that people only need one of any item available. Books, Stamps, Collector's items. There is no reason to get any more than one of each, specially of the latter two. Which means that as stamps and books become more readily available, the number of users looking for these diminishes and thus, the amount of which they sell for. This wasn't a problem in the past due to a lower offer and a higher demand, but now, with a smaller player base and an increased amount of items available to but you can see this is starting to weight down the economy. The best example are stamps, Hoban Stamp was selling for upwards 300k before the charity corner, right now is at about 30k. This is happening with all items but stamps and books are probably the most notable because they've always been known as ETS and good money makers, but now they're coming down like the rest of the items. The only stamps that remain costly are the R95-99, and many of them are dropping too.
A Fourth is the lack of good item sinks in Neopia. The faeries are the best item sink but outside of cookies or the faerie festival, their quests are a rare occurrence, I wish them to be more common (though, I hope the fountain faerie to remain REALLY rare as otherwise I see Paint Brushes prices plummeting, the Grey Faerie should reduce the rate of which she appears). They need to bring back the Kadoatery (The second best sink in Neopia. Would be the best but they only asks for food). Requested Quests are great sinks too, but most of them are abandoned. Jhudora, Illusen and the Underwater Chef are the only ones that regularly see use with Brain Tree, Snow Faerie, Edna, etc... being used only till the avatar / trophy and never to be played again, a better reward (in special items unobtainable anywhere else) would make this more popular. The last sink, which is my personal favorite is the Faerie Job Agency, it's a great way to earn nps but would be better it it had a huge reward to attract even more people in (if it had an avatar for completing 500 jobs or so it would be an amazing sink, even more so than it already is).
And finally we really need a use for such useless items as grooming, instruments and toys. None of them have any use other than be collectibles (or quest items, once in a while).
