Gifting Mon (and Items) is a great way to get rid of what you don't want. The only problem is they usually only come twice a year, so people get irritated about having to look at a Mon for six months or whatever.
A dumping ground would work. Throw in your Mon for 5k, and if someone wants it they can shell out some money for it. HOWEVER, strip ALL Mon of ALL Extra Moves (TM/HM/BM/SM/MT moves). So if that Slowbro has twenty EMs, you remove them all so it's a fresh Mon.
Of course, in a way this is like adding more Mon to the Mart, which will probably never happen lol.
You could do what I do. I have a National PokeDex in which I place Mon I don't want/use. Not only does it get the Mon I never use out of my main stats, it helps fill out my Dex ^^. But yeah, just make a post in your stats saying "Unwanted Pokemon" or something. Dump all of them there, and never look at that post unless you're adding something to it (or removing something). After all, you never know when a Pokemon might suddenly become useful.