I am designing a website that allows people to sell things and send
the proceeds to the charity of their choice. I am wondering if
technology already exists for this purpose.  If not, what are the
recommendations to make it work?

I am not a technical person but having some other company do this. I'm
trying to avoid re-inventing the wheel here if possible. I need a way
to somehow allow people to donate a portion or percentage of their
product to charity.

THis is how it would work. A buys something from B. B receives 80% of
the proceeds and 20% goes automatically to the charity of their
choice.

If this does not exist, are there any charity based web dev outfits
who would help me develop such a mechanism?

it is possible by developing a web program to do it .The recommended
paymnet service would be paypal or moneybookers who allow you to send
email to people to whom you have to pay money(the charity services
email in this case). when the  mail is viewed they will have details
how to claim money .It is fairly complex and requires coding in aSP or
php. for details p m me at vinufelix at myrealbox.com

It seems to me, that the easiest solution is PayPal or any other of
the micropayment options out there.