Actually neoitems removed their prices because of major inaccuracy a little while ago. So that's out of the question.
I think JellyNeo goes by average user shop prices. Look up one of their morphing potions, and those sell in main shops for less than 30k, but the price they have listed is 70k-1.2mil, depending on the item.
Jellyneo has an option to edit their prices, they're all set by users of the website. I change the prices of things that I'm looking up that are inaccurate, but a lot of people don't bother (or don't know they can, either way). The price it goes in the Neopian shops at fluctuates a lot, so I don't think you could really do that... Unless you took an average maybe? Good luck anyway xD
I think prices in user shops fluctuate more than ms. I think would be better if it listed ms prices vs us prices. or maybe add a line for ms prices.
i use http://items.jellyneo.net/index.php as guidance for my shop prices. most of the prices i find are within range of the general price of the item in shop wizard and in other main shops.
At first i believed in Jellyneo's item database, but soon i found it is quite incorrect for two reasons : (1) These prices are uploaded by users, some of them are NOT honest at all (2) A lot of items always inflate/deflate And so, if you trully don't want to lose money, please go check the prices by yourself(by searching SW, TP, Auction), make a list for yourself (with notepad or MS Word or MS Excel) However, item database is extremely useful when gathering items basic information
The lists are accurate for the most part, but the prices should be averaged out or something, and updated every month or so. Also, I've noticed that it's missing quite a few items too, which could be a problem if you're making an AB list.