My response is a big fat NOPE.
If the client isn't happy with the speed of back linking, then it's time for an education session in which you stand up as the expert they hired you to be and explain exactly why it's a bad, bad idea.
- Brokered links are often not contextual, which means they're not actually all that helpful.
- They tend to come from low quality sites, which means they're not actually all that helpful.
And that's just if you don't get caught. Getting away with black hat tactics is an intense, highly skilled, full-time endeavor. It's not a quick and easy way to anything. So even if you are willing to go there, given your inexperience with shady link building tactics, it's still a wretchedly bad idea since you've got no idea how to not get caught.
Best thing to do is to build a content and outreach campaign that builds links in an above-board manner that no one can take issue with. Believe it or not, that's actually probably easier than anything that link buying would steer you into.