I have total opposite problem. They click interested purchasing button, and when I response they do not reply. Why even bother clicking the button!! Of course I report every single one of them to OfferUp as potential issue maker.
On my phone there is a check mark in a circle that you've sent it and it changes to a check mark in a solid green circle when they've seen it. It's easy to miss the difference if your unaware of it changing. Hope this helps!
I am a seller and I have the same problem with buyers . They contact you and want to buy an item and you tell them you still have it and you never hear back from them. It is just as bad as these terrible low ball offers you get or even the buyer that contacts you about an item then tells you they need to talk to their wife or girlfriend first. Please talk to whomever you need to before you contact the seller