Buyers are completely free to message you and offer you a lower price. What is against the rules is if they want to do so off eBay.

I am pretty sure sellers get a radio button at the top of all questions asked that reads "send offer"

I am also convinced that because so many sellers send offers, buyers are getting accustomed to asking questions and waiting for an offer to come in to buy an item cheaper.

Thank you for reading and responding to the message I sent you, via an item you’re selling.

I feel sorry for the buyers that are attempting to get help via Ask A Member, as no one would be able to contact them

Your potential buyer may have read this:

I don’t use ‘make an offer’ but buyers still try……you can either do a special listing just for them, or if they’re ’tyre kickers’ you can put them on your Blocked Buyers List, and ignore them