Spamming can easily be solved by adding a limit on how many people someone can pm at a time. If somebody wants to take the time out to pm 500-1000 people a day I'd say they deserve the business because the competitor who the buyer/ seller wanted to deal with can just as easily match the service if they wanted to. Also advertising by pm to the whole forum is a lot different than just messaging a person who is already seeking a service and implied their interest to the public.
