I'm always suspicious of buying anything for too cheap as I know these films are now almost 20 years old so it makes me wonder why someone would sell an original one-sheet for US$8 while reprints cost the same amount in my local stores (kmart, target etc.).
That's a good question. If I may speculate:
Maybe it's simply not that rare. These were major films with a very widespread release, and very well-received at the time. So not only was there a huge number of posters produced, but people were probably more likely to hold onto these than they would be for some more obscure film that didn't have such a popular reception. Further, by the 2000s, movie posters had been well established as a collectable item. So even though this poster is 20 years old, there are likely a lot out there. And just because a poster is "original," doesn't automatically make it expensive.
Lots of factors are at play in determining value.
And then you need to think like a criminal for a moment: If you were to invest in the equipment needed to convincingly reproduce full-size studio release posters, and assume all the risks involved, would you really sell your posters for $10? I mean... after all that work?
More likely, you have a totally legit seller who has a very large inventory (multiple posters for the same titles), and they want to move their posters more quickly by pricing competitively, rather than pricing it high and having to wait months to make a sale. A legit dealer can make good money by selling in high volume (rather than high prices). This is hardly true for someone who has to make all the posters themselves.
But I totally hear you. "If it sounds too good to be true, it probably is," right? But that doesn't mean there aren't still good deals to be had. On eBay, check the seller's feedback. If they are a fraudster, you'll usually see the red flags. And of course, when in doubt, ask some experienced collectors about a seller's reputation, like you're doing here.
And yes, if you ever do find that someone has sold you a knock-off reprint that they had described as an "original," that is a huge deal. It's fraud, and you would probably be protected by eBay and PayPal's buyer protection, as well as a whole bunch of laws. We don't throw around the term "original" lightly in this community, nor is it used lightly in any other collectables market. The veteran members here would never endorse a seller known to have passed off any fakes as "originals." In fact, that would be a surefire way to unleash the Orcs!