No, I have never used Linkmarket. That being said, have you been monitoring the traffic / conversion data associated with the keywords Linkmarket is targeting? That should give you a really good sense of whether you are getting what you are paying for. Also, have you been auditing where these new links are coming from? I would want to be certain any link building partner wasn't linking in from dubious sites.
Rather than asking people to link to your site, have you considered focusing on the content you are offering on your site(s)? Are you offering content people would want to link to in the first place? There are many approaches to generating this type of content. Consider the following...
- Publish editorial content to a blog
- Create forums for your visitors to create discussions
- Q&A for visitors to ask questions (like this)
- Potentially viral content such as videos, infographics, published research, etc.
I also strongly recommend you watch Tom Critchlow's whiteboard Friday video that focused on Outreach for Linkbuilding - very informative.