You don't have to. Commissions might be the most common case (depending on what they're selling), but plenty of salespeople aren't paid on commission -- I know because I ask them (not when they're trying to sell me something) and I favor businesses who don't pay their salespeople that way. A salesperson working on commission is more likely to rip me off.
For starters when the commission part is a heavy percentage you could just skip the akward process called interview.
If the person is not qualified it won't hurt the company and avoiding the interview process enables for a more organic way to develop a relationship, as the interview is the immediate signaling to the employee that they are a subordinate.