Its cool to go to peoples profile and see when they last posted or when they were last seen. But the option to see when users are normally active would be great in my opinion. Sometimes i have to wait a few days to make a trade simply because schedules conflict and i cant seem to find people online at the same time as me. But if there was something that says user normally active 3 hours from now or w/e i could remind myself to check back rather than just hope i get lucky.

Uhm, can't you just ask the person when he'll roughly be online next time and how long?

Voted no. If it's that much of an issue for you, you can pm the person you're planning a trade with and ask what time frames are good for him

Sorry but i do not see this as something that we really would need.
I agree with Hardytarget and SCXiao

Message the user and ask them.