If the game is in a solid mint condition than it's a good price .
To avoid beeing riped off... well it becomes more and more difficult... with all the converted AES and the bootlegs.
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$400 is a good price for it from what I've seen. I got a complete english AES for $420...
To avoid being ripped off, you should make sure you get pictures beforehand, make sure they're a well rated seller, and especially make sure it has a manual.
Arent Metal Slug X MVS games very difficult to convert? I have heard it isn't as simple as moving chips around, rather you have to burn the game program into a separate eprom because of some kind of protective chip. If this is the case it is very likely an authentic cart unless it is a bootleg, just seems that conversion is unlikely.