Large well known companies have been using this type of advertising for many years by putting logo's,brand names, or symbols etc on obvious display on the outside of clothes etc. It has provided massive free worldwide advertising for these companies.
Now with the onset of the internet, it has become an accepted and reconized form of advertising to attract customers  to websites, merchants advertise their goods or services on the net and pay to have their websites advertised on peoples personal items by putting stickers on cars,  web addresses on t,shirts, pet clothes or anything else they feel comfortable with.

There are of course inherent problems with this form of advertising, such as how can the business being promoted monitor when , where and how often the promotion is happening. The most obvious solution to this is to pay the promotors as affiliates of the business, and therefore it will be in the affiliates interest to attract as much custom as possible because the more sign ups the more the affilites earn .Then there is the question of the materials used, and who incurs thecost? Once these problems are worked out and agreed upon, personal or billboard advertising can work wonderfully well for both the  local pizza shop having an employee handing out flyers in the vicinity of the shop, or for a housewife wearing a shirt advertising a website for a business on the other side of the globe.
There are some terrific examples of this sort of co-operative advertising on the net, EYE EARN is one of the best.