I didnt sell you a tooth... probably couldn't even manage to give proteron a bruise before getting eaten. But if you're giving out compensation could I have some too?
Seriously though I've seen this in other games. Like you said, both parties involved act on information available at the time. Information and prices vary with time. All Sales Are Final. Otherwise people would be crying for more money on everything they sell as soon as mu goes up a percent.
Even if it were the case that you knew that the price would change considerably after the sale. That would be dishonest of course, but is also considered good business sense and shrewd investment.
Unless perhaps you somehow had a direct impact on that price fluctuation either through your own resources, or by conspiring with other investors. In which case it is called "price fixing". It is considered a criminal offence in the real world at a federal level. However, due to the fact it is a virtual world, the value may be under 5,000 dollars (if the tooth your talking about is worth 5000 ped and not 5000 dollars) that it spans several countries, and linking people to an organized cartel in a virtual world would be tedious work- and maybe even impossible if chat records are cleared from MA servers over time, no agent in their right mind would go anywhere near such a case.
I worked in a merchant clan for three years in a game that requires real money for it's items, and nobody including myself were charged with any offence, though we fixed the prices on more items than i can recall, and made a mint in the process... 46M (though 1 dollar equals 1000 gold in game roughly, price is variable and controlled by cash auctioning, and there is no withdraw option unfortuantely

). If you're considered a scammer by getting lucky on a price, then I'm a world-class scammer-extraordinaire. If it was something more than luck, then you and me can just be plain old scammers. Though I do prefer the title "Merch" XD
-edit-
Merchant clans were greatly disliked in that game though, and in most games as far as I've seen. So, if you're looking for friends and you are also merchanting, you will have to give up one or the other eventually... though I made plenty of great merch and non-merch friends and just ignored those that cursed me and my announcements for people to join our rapidly growing clan spanning several mmorpgs.