Fake knives are growing on eBay like an algae bloom.
There are two kinds: copies and counterfeits.
Copies are designed to resemble a well-known, high-quality knife, but do not actually claim to be the same. There are copies of many models of Microtech, CRKT, Uzi, Spyderco, etc. If the knife is vague about its origins and surprisingly cheap, it's probably a copy from China. All the copies from China use the lowest-grade materials, and the blades will not hold a good edge. Almost every well-known knife has an Asian copy on the market. They are worth less than you pay for them.
Counterfeits are knives actually claiming to be from a reputable manufacturer. Right now on eBay there are many of these. If you search for Cold Steel AK-47, you will find some that start around $10. Look carefully at the texture of the handles, and then compare it to Cold Steel's website. You will see that they copied everything except the handle texture. Also, the grind on the blade is not straight. Definitely a counterfeit. They scan the boxes of the originals and print their own, and take molds of the knives, so they look very close.
Ways to avoid getting ripped off:
1. If it sounds to good to be true, it is. Good knives made of great steel cost money. For $10 or $20, you get nothing but crap.
2. Know your manufacturers and models. You will learn to spot the subtle differences. Check manufacturer's websites and look closely at the pictures.
3. You are not going to get a good name-brand knife for a few dollars from an overseas merchant. They pay the same wholesale costs as anyone else, which is about half the MSRP. There is no way they can sell original knives for a few bucks. If you see a Cold Steel, Spyderco, or Dark Ops knife for a fraction of what everyone else is charging, you can be certain you are looking at a counterfeit.
4. Always deal with a reputable knife dealer!