The burden of getting a startup off the ground is centered around engineering, design, and actually getting your hands dirty and building the product.

As an engineer, I've been approached by those experienced in sales and offer me to build a mutually agreed upon product that we agreed would make money for equity. I would like to know if this model works. If so, how does it work? Is this common?

You can take offense if you like, but if you meet a guy at an entrepreneur conference who already built a prototype, you're not a co-founder.