If I get a nod I nod back because I think it's the polite thing to do. But I think there should be a nod and a wink option. Some people see a nod as a general hello but others see it as an invitation and the wrong sort of invitation.....

If after someone nods and I nod back they then send me a message I also reply. I don't mind chatting to anyone but unfortunately some people take it as a come on and sorry but that's not on.
A nod and wink option would mean you could put you intent across without either having to end up being blunt and upsetting people or blocking people.

So if someone winks at you but you're not interested like that you could nod back, or if you think "aye aye you're a bit of alright" you can return the wink, then everyone knows where they are. I just hate giving people the wrong impression or seeming rude. And now I'm rambling, but think I've explained myself