I found this neat little article in the ebay guides. Not exclusively related to sealed PS2 collecting but it is good, sound advice about collecting and ebaying any type of sealed video game.
Some video games (especially the older 'retro' ones) can certainly go for quite cheap, even if they are in mint condition complete with box and manual. However, the value of a game can increase 1000% if the game is 'factory sealed' - these can be worth their weight in gold to collectors. But just think - if people can buy a complete Legend of Zelda for 20 bucks, reseal it and sell it for $500, well, thats too much of a temptation for some. How can you tell if someone is selling you a resealed game?
In this case, Nintendo has the infamous 'horizontal sealing line' across the back, but this is certainly not impossible to reproduce with a little practice. So, what else can you look for?
Common sense ought to be your guide here, so look at the overall picture:
What does the shrinkwarp look like? Is it neat, professional? Or are there sloppy, loose corners? Lots of wrinkles?
Is there a price tag? Do you recognize the tag (Toys R Us, Babbages, KayBee...)? What shape is the tag in? Are there signs of wear or fading, or does it look like it was printed yesterday? Does the pricing on the tag make any sense? For example, a game might go for $51.00, or $51.99, but who has ever seen a game go for $51.82? Are all of the seller's games showing the same price tag? While its possible that the seller did get all of their games from the same source, taken in to account with other factors it can be a little suspicious.
Is there a hang tab? What shape is it in? Are there signs that it was ever used to hang the game? Does it look its age, or does it look brand new?
Try to get a good look at the edges and corners of the box beneath the shrinkwrap. Are there any signs of wear or rubbing that would indicate that that box has had some use prior to being wrapped? The 'hinge' on the lids of boxes is an excellent place to check, and the corners along the bottom also.
These are just some things to think about. While it is entirely possible that someone 25 years ago had the foresight to seal NES games in a vacuum bag or similar to protect the price tag/hang tag, or that some local store accidentally marked games at $42.85, or that Nintendo turned out a few sloppy sealing jobs - the odds are not high. So, also consider your seller:
What is the sellers feedback? If there are any negatives/neutrals - does it concern fraud? Keep in mind that if the seller is limiting his cheating only to sealed games he may never be found out. Most people will not spend $200 on a sealed game and then open it just to see if it looks tampered with, or kick off a foreniscs investigation to see if the box has ever been opened..
Check the sellers buy/sell history. If they have bought a shrinkwrap machine or a price tag maker, you might want to be a little more careful when dealing with this seller. If they have just bought the same games (unsealed) as you are now seeing for sale sealed...well....you have been warned.