Boy, does this hit home for me, on two levels.
First, my mother is a hoarder (the clean version, and not too severe yet, but still a hoarder) and foists stuff on us regularly. If I refuse something strongly enough, she will take it back, and then hoard it with the intention of 'selling it' (they do have a regular stall at a swap meet, but are so disorganised and unrealistic they don't move much). She regularly tries to elicit promises from me that 'I won't just throw it out' as well (I usually do; so far, she hasn't noticed). This is partly because she loves me, and partly because she simply wants to 'extend' the hoard to our place. This is often the case with these kinds of loving donations, so beware - you are often unconsciously viewed as a self-storage facility.
And second, as a volunteer for charity, I help organize a massive (we make in excess of AU$5000) annual jumble sale. We get tons of donations, many of which are perfectly reasonable if not spectacular. But we also get a lot of stuff that is useable but simply isn't saleable (and sadly, some rubbish in the literal sense of the word). (I have a good working knowledge of antiques and access to Ebay as well as several years' experience, so I do know what the items are worth.)
As CakeEater and a few other posters have pointed out, many people seem to lose perspective when the items are something that were once valuable to them, regardless of value now. In the case of the jumble sale we tried refusing certain items, but people got so angry and complained so much that management said we have to thank them nicely and just get rid of it. Luckily, we rarely get huge items like furniture, but as we operate on a shoestring budget this means I, and the other volunteers, are hauling huge bags of junk home to put in our own bins once we've filled the skips (its' expensive to hire more). It just seems to me that it is more about the donor's feelings than benefiting the recipient.
When people talk about donating to charity, I usually tell them not to donate any of the following, even though it may seem wasteful: videotapes, cassette tapes, encyclopaedias/general nonfiction more than a few years old (like travel guides), magazines more than 3 months old, mismatched plates and glasses (again, nod to CakeEater), electronics (unless the charity is specifically asking for them, it is against OHS), bags and shoes in anything but very good to brand new condition, clothes that look 'tired', mattresses and pillows that are used, underwear (yes, we get cross hatched underwear
), toys that are missing important parts, and stuffed toys in general. These may all still be useful, but they don't sell
. I'm sorry.