I would ask the people who had ordered the skip before taking anything, as it is theft here, as by law we own our rubbish until it has been collected, when it becomes the property of the company removing it.
Formerly, I lived in a block of flats that had a shed we put things into that were too big to go in the bins - this is common in Denmark and the landlord pays extra to have these sheds emptied once a month or so. Although strictly speaking it wasn't legal to do so, the residents of the flat were in agreement that if you saw something you could use, you might take it,
In Germany, I have seen householders when putting furniture, clothes etc out for collection sometimes write a notice saying, "take anything you can use", Whether this strictly speaking gives you a legal right to take it, I do not know, but I presume it amounts to you having spoken to the householder before helping yourself.