I've sold MF previously on eBay, and people snapped it up, sometimes close to the official price, sometimes quite cheaper.
But would I buy it myself there? Only if the person had great feedback and their home was NON-SMOKING. Gag! I once traded with someone off a MF support forum (not 3FC) and never thought to ask about the smoking habits of the residents.
The packets were incredibly tainted, even though they are metallic-foil lined! I tried one to test it, despite the lingering stink, and threw out all of the rest after one sip.
I didn't send them back and make a big fuss because it took me about a month to get them from her -- with the delay caused by her brother falling ill from a infection that went septic and having to go in the hospital -- and her being too involved in that to mail me the MF.
That was my last trade
Just be careful in selecting your sellers I'd say and yep, check those expiration dates!