Buyback here I mean people can post on WordPress saying they want to buy from you say your old iPhone 9 for USD500. It's like a product request feature. I googled and can't find any.
That does not look like WooCommerce to me. In Woo, you list specific products for sale, maybe with some variations. And for those, if you like to make an offer, there is, for instance, Name Your Price extension.
And if you say "want to buy your old iPhone 9 for USD500" - then who is "you", the phone owner in this case? That looks more like a classifieds request.