St Georges Charity shop on Perry Hill is always grateful for donations. It is the most chaotic charity shop I have ever been in though, and the opening times are a bit random, but whenever I have bought anything it's always a quarter of the price of other charity shops.
I do agree charity shops have to put up with a lot of junk - I've seen stuff just dumped outside the charity shops that is quite obviously only suitable for the bin, who on earth would do that? It's pretty sad.
Otherwise I have found Lewisham Freecycle is a good place to get rid of baby items that it's difficult to sell. I think sometimes you just have to accept you won't get your money back on these things.
The Foodbank on Malham Road behind the Fire Station sometimes needs donations of particular items like towels, mens clothing, but you need to pop in and check with them what they are short of.