I don't think any offer is offensive, a seller can just reject it or accept it. Though as a seller, I am annoyed and pretty quickly reject an offer when it is less than half the asking price, especially on a sealed record. If 2/3 makers sense based off of the sales history, condition, etc. than it's a fair offer IMO.
I think between 10-20% is usually what's acceptable. I look at Discogs median too, so if it is sky high, but has a make me an offer, I may go below the 20%