When you have the opportunity to buy a kit like this, you must decide whether you are going to build it (even if that build may be years in the future), or if you are buying the kit as a collector, and you plan on leaving it in the box.
If you plan on building it, then the fact that it's a kit in its original wrapping already drops in importance as a criterion, especially if it's a common kit, that was turned out over many years. For example, if I want to build Monogram's SBD, I know that it was issued in the thousands, over many years and many changes in packaging. So, if I see a seller offering an original issue for a high price, I'll pass and look for one for sale for $5 in the white box from the Seventies.
If it was a rare kit, then the fact that it's in its original packaging, unopened, shrink-wrapped, might carry more weight, but I will still want to haggle the seller down from a collector's price to a builder's price. Again, if I'm going to build it, I'm going to open it, so the packaging is not as valuable to me as knowing whether the kit is complete. And as Rob pointed out, there are unscrupulous folks out there.
If I were buying the kit to collect it, then whether it's unopened is more important. I would want to have educated myself before even entering the market, by picking up and reading everything I could get my hands on about kits, whether in hard copy (eg, Thomas Graham's books on Monogram, Revell and Aurora are excellent references) or online; and I would watch sales, such as on eBay, but also at shows, and at live auctions, and get an idea of values and asking prices. I've done that for other things that I collect, such as toy soldiers and beer steins. Also, bidding/buying and getting burned is also a lesson, and I don't think there's a collector out there who doesn't have at least one story about getting flim-flammed, or even just making a dumb purchase (I have a couple).
But in this case, it sounds like you might build it, so, open it, and if it's intact and you didn't pay that much, you made out pretty well. And show us your build!
Hope that helps, best regards,
Brad