I got a message about selling worldwide (I don't currently do that), and when I tried to respond, it wouldn't send. He wasn't an etsy address, or something along those lines. If he wasn't logged in, I don't know how he sent a message. I don't know if it's spam or this is a glitch. I sent it to spam because I don't want to lose my star seller ranking. Thoughts?
He was probably already caught as a spammer/scammer and Etsy deleted his account. Where you put the message is right where it belongs.
As above, a scammer Etsy dealt with before you had a chance to reply.
If you got an error message which said something about an 'invalid user' then the account has already been closed (probably by Etsy). They could have been spamming or a scammer. Marking as SPAM was the right thing to do.
@kickglassenamels: "He wasn't an etsy address, or something along those lines." If the message is something like "User Name is Invalid" usually that message means that the account has been closed (either by the user or by Etsy). Lately, in most cases that will mean a scammer was stopped. Just in case, you should make sure to mark the message as SPAM (at last report you cannot do that using the seller app only a browser, even your mobile browser, or the buyer app).