Welcome to the community, @MrsDi! The only thing you are "guilty" of is looking for a good deal. While lowballing may be annoying to some, it's just one of the facts of life when it comes to buying/selling. Sellers want to make money, and buyers want a good deal. While I don't sell items for lower than my asking price (for the most part at least) I can't fault someone who may be on a budget that is interested in my item. Doesn't hurt to ask, imo.
I don't think you want to compare Offerup to your local department store. If I walk in, for example, Walmart, I expect the item I purchase to be new, have a warranty or guarantee if the item does not meet my expectations. Do you offer this for your buyers? From time to time department stores will issue coupons or offer discounts, too. I'm sure when you shop at a large store such as Walmart, Target, or Sears, you go looking for a good deal. Well, it's no different for me when I search on Offerup. Several weeks ago (to my suprise) I was given a low rate at a hotel because I asked.
I don't know what you mean by "feel so entitled" When I asked the seller for a lower price, I came looking for a good deal. I knew what the asking price was too high. Had he countered my offer, I woul probably have purchased it because I really wanted it.
You mention, "How is it fair for the seller" You are not obligated to sell your items to anyone who lowballs you. If it were mandatory for you to do so, THEN it would certainly be unfair. If someone lowballs you, counter offer or simply do not respond.
"90% of sellers have no car" I am not aware of those statistics. But, if the buyer wants you to deliver, there is any easy fix for that, DON'T. There is no need for you to play UPS.
Sentimental value is placed on items one has become attached to NOT new items. And, yes, it tends to be very high because the item has been passed down from several generations. If you have something you grandmother gave your mother but no one has used in for 20 years, don't try to sell it as "Like new".
"Expecting half off or More" If buyes are doing that then set the price of your item for more than half of what you would like to receive...you want $25 ask for $50 or more. It is not scheming, it is looking for the best deal. You said, "90% of buyers have no car"...maybe these 90% have almost no money.
No, buyers should not be banned for lowballing. Learn to negotiate. there is no rule stating you have to accept and offer. Make sure the pictures you submit are clear, and take more than one. I have seen sellers show only one picture of an item and it's out of focus. This would lead me to believe the item is not worth the asking price. Large retailers have to negotiate contracts, and you better believe a lot of lowballing goes on
Do not understimate the buyer as being broke.Yes, they might use tactics you do not like, but they certainly can't get there way if you don't let them.