"Everyone has at least one story of the weird guy that turned up at their house to pick up the stereo. We wanted to make something that enabled people to be able to sell within their own networks," he explained.
"It operates on the idea of a social contract - you're not going to rip someone off if they are within your own network of friends."
He said the support of Dotcom, which came after a chance meeting, had been "generous and priceless".
"He's such a nice guy," Campbell said. "We've got huge capacity on servers all around the world, but I think we will be holding our breath if he tweets it to his followers."
Dotcom has collected nearly 137,000 Twitter followers in just a few months on the social network, and his tweets, in turn, are often retweeted to tens of thousands more people.
The Buddy Bid tool has already been discovered - without the involvement of publicists - by Grattan McCoach, the UK managing editor of Huffington Post. He put a second-hand baby buggy up for sale.
The entrepreneurs were yet to make money on the tool but said there was potential to charge a fee to businesses.
