Well this site was made and directed towards RuneScape cheating so most of the MiddleMen here MiddleMan Runescape trades. Since I'm guessing you are here for Xbox Related Sales there are a few people I would recommend. Probably your best bet would be Feren Silver, he's on his way to becoming an Official Middleman and he Middlemans Xbox Sales. He's who I use primarily. Another good MiddleMan is djweasel, cool guy and great MM
I also Middleman, not to be too self-confident, I've worked with lots of 50s and General accounts, I've also worked with recon
Those are just 3 people though. On occasion people will ask for an Official Middleman. They charge to Middleman but you are basically paying for trust and safety as you are almost guaranteed not to be scammed.
Basically, just find someone who you trust with your accounts and money and they'll probably be a good candidate.