I agree, I'm afraid. Tell her that you have made your best efforts to sell it, but cannot, so if she doesn't collect it by x date, you will get rid of it.
If it leads to a torrent of verbal abuse, honestly, you need to stand up for yourself.
"Mom, I tried to help you out by selling it. I am sorry it is not worth anything. Please don't shoot the messenger". I would just repeat that over and over every time she mentions it, but I am kind of a bitch. If you are more embroiled in her (what honestly sounds like narcissistic) behaviour you may find that hard / impossible. But I think it's good to start establishing these kind of boundaries (ie you can't ask me to do you an impossible favour, and then berate me for not doing it) because, IMO, people like this tend to get worse as they get older.
I agree that giving her cash and lying, whilst very appealing, will lead you further down this path which you do not want to go on. I know it's great that she's letting things go, and it's good to encourage that, but, honestly (this is going to be direct, sorry if it's harsh) if you are trying to encourage her to let stuff go so that one day it's easier to deal with her estate, it's not working, because, when that sad day comes, you can donate it all without anyone's agreement. Doing it now means you are having to negotiate with her.
Sorry she's being like this.