Any company that sells links isn’t reputable.
I wouldn’t go that far. Any company that sells links and doesn’t “nofollow” them is either dodgy or incompetent (they might simply not know that they are doing anything wrong, you can’t always attribute to malice what could equally well be stupidity).
But there are plenty of legitimate reasons why a site might sell links, but they should be clear about the purpose of the sale and what benefits the buyer will/won’t get. eg, they will get their site advertised at the top of a directory site that gets x thousand hits a week, but that link won’t give them any extra googlejuice.