The FBI may not be able to take down the entire store, but they don't have to.
And the second issue: what if cops traced the package to a country where selling these drugs is legal (e.g. generics in India)?
For the second issue, I guess they wouldn't do anything (though I'm pretty sure selling drugs to someone in a country where it is illegal is always illegal). But unless the only dealers on there are doing that they can still pretty easily catch dealers. And, even with a perfect reputation system, once you catch a dealer you can locate the addresses of all the customers he sold to and bust them, or keep his online business running for a while and busting buyers until his reputation runs out.
EDIT: actually, does Ethereum even allow for hidden information between two parties? Isn't it just a public ledger so everyone can run the code and verify? That means anyone could get all the shipping addresses
An account (seller or buyer) with a positive reputation could be hijacked if their computers are compromised or they reveal just enough PII for an investigative team to track them.
Re second issue: It may be legal to sell (drug) in (nation). If it's not legal in (other nation), then you're breaking the law by sending it to (other nation) even if you're in (nation). Local legality does not protect you entirely, though it may shield you partially (through your nation not supporting extradition). But should you enter (other nation), be prepared to be arrested (this has happened several times in the US).
The protection for sellers is that it's actually pretty hard to track one down from a package. Not impossible but hard. (And you might even get the wrong seller. The seller you buy from might just be buying from someone else and entering your information for delivery, like drop shipping. If you're concerned about a seller, you might even pay a premium to sell their same item at a loss and wait for someone else to take the buy risk for you.)
It's interesting to look at known dark market arrests, buyers tend to go down to controlled deliveries of certain items but there's not all that much commonality in arrests. https://www.gwern.net/DNM%20arrests#analysis