When sellers are seriously considering taking the items and want them for themselves in their new house, they simply state that the items are not available.
Anything they are willing to offer for sale or negotiate on, they don't really want for their future place. Yes, some will think it's a good chance to recoup some of the expense if the item from when they bought it or compensate for accepting a lower offer on house, but forget that secondhand items have usually lost the majority of their value.
So if they won't accept £200 when they wanted £1000, they may well find they either have to leave the item and receive nothing (as its too much of a pain to remove) or they take the item because they don't want to give a 'freebie' and are then stuck with an unwanted item that at best can get £50 if they can be bothered to try and sell it. Either way they would have been accepting the £200.
And when it comes to actually moving and all the stress and stuff to do, many people will leave those curtains then, even if they only actually decide to do it at the last minute, because they can't be bothered to take them down when busy on moving day.
Best thing is not to sound too bothered and not to engage in lengthy must-stage negotiations. The more the negotiation goes on for and the more the seller feels aggrieved about buyer not paying what they have asked for, the more likely they are to be bitter and take the item for spites sake, if you know what I mean.
You can never know for sure if you would have been left the item without paying or if they would have accepted a lower offer, but you can give yourself the best chances by assessing the ease of removal of the item and 'making good' effort and essence they would incur, plus playing it very cool and not seeming too bothered about the item when engaging in one quick offer. Lots of offers and negotiations certainly gives the impression you really want the item....and they might hold out for more.