I searched the words smooth seven and everything on urban dictionary but still couldn't figure out!
When you pick up trash, you pick it and dispose of it properly, like in a trash can. However, pick up also has another meaning:
[transitive] to go and meet someone or something that you have arranged to take somewhere in a vehicle
Will you pick me up after the party?
I need to pick up my luggage before leaving.
It is often used to confirm a romantic date, as in
How about dinner tonight? I'll pick you up around seven (o'clock).
In other words, the second comment was a pick-up line used to ask the other person out on a date. "Is seven okay?" means "Is seven o'clock okay?", or "Can I pick you up at seven o'clock?"
The third comment, "you smooth fucker", is acknowledging that the other person was skillful, suave and that the pick-up line was clever.