i just wrote a reply and it disappeared... I'll try and summarise...
I buy and sell a lot, and go in and out of a lot of homes as well as have people come to me.
There are companies that arrange sales - take a commission, put security on the door and promote the sale, though if you dont have much to sell they are unlikely to be interested.
craigslist - i have seen some very slack security - i wonder how these people have not been robbed - one paraguayan girl left me in her home while she opened the door for someone else, just after putting money in a drawer, in front of me with no precaution.
She took a silly risk and i would not be surprised to hear that someone stole it later.
When i have people here and i have to go down to answer the door to someone i ask them to wait in the hallway. Nobody gets offended, its just a sensible precaution.
I have had no negative experiences, touch wood, and i doubt you will, but be careful all the same.
Oh yea, and the mercadolibre thing - you can not control who buys something - a user with no reputation can not be blocked. Obviously you can refuse the sale afterwards, but then you'll end up with negative ratings, and in turn, no buyers
good luck, let me know what you're selling too!!