I've been offered 100% commission jobs in the past. Most of the time, it's the owner of a business who makes the claim that if I'm as good as I claim to be, I shoud be willing to take the risk.
To date, I've yet to accept such an offer, and I'm about to write some reasons why. Maybe it will help you the next time you get such an offer.
1) A salary is a risk: Owners know that hiring someone is a risk. And it's a risk they bear the brunt of, because the employee is protected by a number of laws once they've signed the paperwork. The genesis of a 100% job is an attempt to split that risk. It's a valid argument.
2) Taking risks in business is supposed to lead to a reward. Considering the business owner takes the risk of hiring you, they should get the benefit of the reward when you make them a lot of money. If the business owner wants to split the risk, you need to ask if he is willing to split the reward. Will he give you 50% of the profit on the sale you make? Will he double or triple commissions? Will he pay a residual on business? If he's comfortable getting you to shoulder some of the risk, he should be comfortable giving you some of the reward. If he's not, he's just trying to negotiate a no-risk hire (which makes you a bad salesperson for accepting it).
3) What kind of sales cycle and sales support is needed? If you're selling a product door-to-door and it's a one call close, this is an easy decision. What if you're selling a service, or a high dollar product that requires additional technical expertise? Part of the reason to pay a salary is to pay you for work you do. If a long sales cycle is involved, not paying a salary bring a greater risk to you, as you're ultimately not in charge of your own destiny. You really on others to help you, which is not the role of a 100% commission sales rep. If you're not closing a deal on your own, you're setting yourself up for failure.
4) Are there restrictions when you leave? The worst thing a business can do to a 100% commission sales rep is restrict them with non-competes or non-solicits. They're not paying you, and yet they claim to have ownership of the work you do? Run screaming from someone like this. They're not interested in a profitable relationship so much as a sucker.
5) Getting paid when you close, not when the client pays. It's always best to get a check when you close a client. That's harder with bigger companies, but if you're left waiting 30, 60, or 90 days to get paid, you're the salesman, and the banker for the company. In addition, if you leave, you run the risk of not getting paid at all. It costs more to get paid after the lawyer, and that's if you win. And the loss of money in dealing with the lawsuit will likely be greater than what you are owed.
6) Who pays for overhead? From offices, to cell phones, or marketing collateral and business cards, selling can be expensive. If you're paying for all of it, you need to make sure that it's not eating into your profits.
7) Are you being managed, or required to do busywork? Finally, if you're on commission, you should be your own boss. Attending sales meetings to pump you up, or filling out paperwork or timecards, or having your numbers checked or data entry required isn't part of the deal. You're either on your own, or you are with the company. Businesses trying to skirt this are breaking the law on how to report employment, and you need to make sure you understand what 1099 or corp-to-corp really means.
In today's climate, you sometimes have to take what you can get. When first starting out, or if you really can sell, 100% is not a bad way to go because it offers you experience and freedom. On the other hand, taking a job that costs you money is a hard thing to swallow. Be vigilant, and remember to use this line, "What kind of successful salesperson would take that kind of deal?" It works because it's the truth.