Those that know how to build links are usually doing it to benefit themselves. It's usually people who don't have the time to run their own website's who offer services as you have described. It's a mix and match, you'll find that there are those here who do have the knowledge required to partake in such a request that you posted, but you have to expect responses from those who are trying to blag their way into making some money. I'd recommend being more specific in how you place your request, and lay down some guidelines for those who are wanting to apply for your task. Following that, continue to check the iTrader of those who apply. At the end of the day, it can be very hit and miss, and to ensure you get what you are looking for, either outsource it to someone offering this service as a long term idea (e.g those who actually partake in the action of building links as a career) or do it yourself. Sometimes, just allowing yourself 10-20 minutes a day just to link build will give you enough insight into the job you are asking to be done.
I actually think, by reading most of the responses here everyday, that most members on DP know very little about SEO. If they can show me a website of theirs that is ranking in the top 5 for a significant keyword or phrase, then I'm sold. I'm not going to take bodybuilding advice from someone who has 6 inch arms, even if what they know is facts. Link building is one of the most tiresome, frustrating, yet rewarding jobs out there. It is slow and tedious, but when you experience growth it usually last for a while. My suggestion: buy a recently published book about SEO by a real expert. Don't believe all the nonsense you read posted by absolutely anybody on these forums.