I wouldn't suggest trying it

. Not only is it a very hard thing to do, but it's also against the rules and could end up getting you frozen or all the items you bought deleted.
QUOTE
Q. My friends and I have been purchasing all the Dr. Sloth Costumes and then jacking up the price. Is that okay?
A. No, you cannot conspire with others to corner the market in an item and then artificially raise the price. This is called price fixing which violates Neopets' Terms and Conditions. If we detect this, we will either freeze all the players involved in the conspiracy or delete all the items they are hoarding. However, in anticipation of a war or other event, you can, by yourself, purchase unlimited quantities of an item in the expectation the price will rise. This is called price speculation, which is fine.
Found
here.