We've seen these things on eBay. Countless numbers of them. CHEAP USMC NCO SWORD BEAUTIFUL, are usually the title's of the auction. Your certainly not going to get a good one for if your going to spend less than $100. However here are the major differences.

Tassel:
Clearly the maker's of these swords do not know the Marine Corp. The replica I purchased came with a tassel. Marine NCO's do not wear a sword not or tassel on their sword. However Marine Officer's do wear a leather Sword Knot but it does not remotely resemble a tassel.

Metal Colors and Screws:
The color between a real USMC Sword and one of these replica's is truly different. The replica is usually to yellow in color. It also has a screw for an end cap instead of a nut, whilst the handguard is also improperly proportioned in shape. It also does not meet the required size specification. That's why it always feel weird when carrying it.

Scabbard:
The scabbard has many faults. Often covered in vinyl, while the seam is on the edge of the scabbard not along the backside as required by the official USMC specifications.

Blade:
This is where the worst problems are encountered. The word United is often mispelled as Unued or so it seems because the letter T is so small it's almost indistinguishable. The level of detail is also very bad. You can not even the the United States Flag or the Eagle, Globe and Anchor that is supposed to be etched into the blade. The blood groove also seems to be cut into the blade, instead of being forged. And since it looks like they first "etch" the blade and then put in the blood groove the whole job looks sloppy.

Remember know. Next time your going for a USMC save your money and go get a real one.

Sources:
Marlow White US Military Swords. < Best Marine Corp swords I've ever seen.  http://www.militarysabers.com/buying-guides/cheap-sword-replicas.html

Toledo Swords <This is the best place to purchase a "cheap" USMC Sword.
http://www.loveleaf.net/sword/mil.html