A few months ago, a company approached me and offered to buy a textlink from my site. At first, I was sceptical, but the company turned out to be reasonably professional and the website I have to link to is decent and related in topic. Thus, I thought I could expand the collaboration - but they have a limit on one client per website only.
How do I identify "serious" companies like this one in my endeavour to get advertising partners for my websites? There are thousands of companies of this kind out there, but I don't know how to find "respectable" ones. Any advice is appreciated.