I don't think charities will be able to sell copies legally either, they're likely to be sold as used blank tapes if at all (that is a bit of a grey area one could argue over - 'sold as blank', though it's probably still illegal in the law books and I certainly wouldn't try it in any online auctions).
It's like the old favourite I keep seeing in online auctions - a computer loaded with software but no original licenses or media - completely illegal. If the seller makes no mention of the contents of the hard drive but there happens to be lots of software installed (without physical media or licenses included, which are sometimes non-transferable anyway) then I believe it becomes the buyer's moral dilemma as to whether or not they delete it.
If you wish to remain 100% legal then just record over or destroy copies you no longer own the originals for, or you could re-acquire originals (e.g. legal downloads from an online music store), but to be honest I don't think anyone's going to come knocking on your door to check.