We have twins & so have four Britax convertible car seats that we recently had to replace b/c they outgrew the height limits. They are in great shape, have never been in an accident and they don't expire for another year or two. It seems like a crime to just toss them and I don't even know if typical donation places even take carseats b/c of the risk of them having been in an accident or something. I don't know anyone personally who needs this kind of seat.
The other thing that is kind of random about them is they are Britax Diplomats, which were discontinued shortly after we bought them b/c they are actually a smaller convertible (which is what we needed so we could put one rear facing behind my husband when he was driving, the full size was too big for him to sit comfortably). I assume they discontinued them b/c the new 2 yr RF recs were about to come out at the time and with the lower height/weight limits some bigger kids wouldn't be able to be in them for as long... (35 lb RF, 40 lb FF, 40 inches for either) My kids are on the smaller side and were over 4 before we swapped them out so it wasn't an issue for us.