FB Marketplace- Add LINE?

[recom273]

I sold a couple of bikes of FB marketplace in the past, it wasn't a great experience, a lot of people asking if I still had the bikes and then asking the price - which was clearly on the ad, then never responding. I had a lot of people asking for discount before even seeing the bike (which was reasonably priced for a fast sale). Out of so many discussions, extra photos, and questions, one guy actually appeared. 

 

I saw a couple of marketplace posts, from westerners I presume, saying, no discount, no I dont want to add LINE, messenger only.

 

Now I am back on fb marketplace selling small stuff - idk why? cat beds that were bought that they would never use - 100B each - etc .. we get enquiries within mins and no reply, then forever "add LINE" - Can anyone explain why? and why do some people not want to add LINE - I prefer not too, it drives me crazy. Is there an associated scam or can business be conducted outside of marketplace or something, idk?