You need to be real careful suggesting that because Marcos are often sold as ghost shrimp in places like Wal-Mart and even your LFS, and they will eat your shrimp. I have a 5 gallon that is now devoid of cherry shrimp and 2 really fat "Ghost Shrimp" as they were labeled.
Sorry to hear about that... I know it can happen- but I don't think it is common. I know mine arn't macro because they are berried and are still small.
I don't think any store is going to sell a $5- $10 shrimp for 30cents if they can help it. It's not going to be deliberate- but an accident or act of ignorence on their part.
Still, if someone is worried- look to see if the shrimp in the tank are berried. If they're an inch and a half long and berried- that's too small to be an adult macro- so they're Freshwater Grass Shrimp most likely...
... still, I suppose it is possible to have both in one batch.
Personally, were I to find out my ghosts were Macros (I know they're not since they're adults and still look like ghosts), I would be over the moon that I got so many so cheap!

- Of course- I don't have any other shrimp in the tank yet- they're coming later and I would be annoyed if my ghosts ate their tank mates.