New to poe.xyz trading, need some help

When I search for stuff on poe.xyz, I always search for people online. I've come across several cases where I've whispered someone who's marked as online by poe.xyz and got a replay saying "@<-That character is not logged in" I know I typed the write name because otherwise it'll say something like "that character doesn't exist"

So for those experienced traders, could you tell me what I'm doing wrong or is it just the system's fault?

That happens sometimes.

Go to their shop, click on their profile and message all their existing toons. It has worked for me in the past....

Nope, the problem is that they have clicked themselfs as online on the poe.xyz.is website, and it'll show them as online for 3600 seconds/1 hour and they (me included) have to do this every hour to be showed as online, even though they have closed down the game they'll showed as online. So I suggest sending a PM if they're offline.

that whole online thing is very sketchy.

besides the fact that they aren't always online, it will only show them online if the char happens to be on th eladder -- meaning some min lvl like 75, and they happen to be on that char.
it's really almost entirely useless.

I'd suggest copy/pasting the name to avoid errors, and using the /whois command and/or adding to friends list.
ie /whois eommage will tell you if I'm online and what area I'm in, so u can see if I'm in a map, etc

when added to friends you'll see if I'm online, but if you do this a bunch I suggest keeping a txt note and pasting the name in there with item and price for ez reference.
you can also pm the guy in question