You value them based on what somebody is willing to pay. So, you ask these folks to make you an offer. If it is more than the £450 you're looking for, you can

You value them based on what somebody is willing to pay. So, you ask these folks to make you an offer. If it is more than the £450 you're looking for, you can take it, or hold out for more, if you think they have deeper pockets and desire. If they turn around and sell it on for more, that's no loss to you; you got what you thought was a fair price, and weren't willing to put in any additional sales effort.