I don't know I would ever block a buyer just because they put in a low offer, after all, it is up to you whether you accept it or not.

Just say no.

They can only put in 3 offers max, so if they keep going too low they will block themselves.

I am afraid though that i find offers of 50% on the insulting side.

I strike it constantly on gumtree. I will have people email a message-I can pick up that bookshelf today, will you take $30 for it? (advertised as $65, my sister ended up selling it on ebay a day or so ago for $77 )

I felt very snappy with the low offer and the casual way they seemed to think it was a fair thing.

In fact, making an offer is so common on gumtree that I do it myself now, but I always start by saying the item looks nice, is the price at all negotiable. That leaves it in the seller's court, but if I did make an offer, I would probably make it more ball park. For example, I did have one man offer $55 for the bookshelf and i accepted but then he messaged back he had measured up and it was too big for the space he had. fair enough, but the point is, for a $65 item, at $55 I was not insulted, at $40 I was.

I think 5-10 % off might be more the ebay range of seller discount, although some may go to 20%. I would say that 50% would be unlikely and I would never type that in as it would feel like a waste of an offer.

If you only have 3 chances you want your first offer to be maybe possible, not clearly out.