Definately way overdue IMHO to block sellers and ALL of their listings.
These sellers who are spamming OfferUp are ruining it for everyone. When a buyer comes and see's your front page filled with numerous of the same item they often move on to alternative options.
Please also along with people being able to block a sellers listing, do EVERYONE a favor and keep track at OfferUp of how many people have blocked that seller/person and take appropriate actions.
It is utterly ridiculous that I can't search for, say, "Verizon" without my feed being overwhelemed with business posters. I've even tried flagging the posters and blocking them; nope, feed still inundated with their spam.
OfferUp - pay attention. You're going to lose users over this.