There is a chance, albeit a small chance, that this person is on the level but I wouldn't count on it.
The e-book publishing business is far from a professional field. Any idiot that thinks he's got something to say can publish an e-book. The barriers to entry are a lot lower. Like digital photography, digital publishing attracts a lot of wankers who only know enough to be dangerous.
This could be one of those wankers or it could be a crook. In either case, you should still be careful. Make the person cross every "T" and dot every "I." If he does anything that is the slightest bit hinky, call him on it. If you don't like the answer you get, tell them to stuff it where the sun doesn't shine.
Who knows? This could be a genuine opportunity to make a buck. Just don't let that opportunity cloud your judgment.
As I said, before, I would get a land line phone number to call them back on. I would get a physical address that's not just a postal drop or a rented office cubicle. I would make them put everything down on paper, hard copy, and sign it with pen and ink.
Look at it this way. It's not like you've lost any money if you tell them to take a hike. You never had any money in the first place but, if this is a crooked operation, you could stand to lose a lot more than money.
Treat this as if it COULD BE a genuine offer but look at it with a jaundiced eye.
BTW: I have an idea...
Instead of a straight percentage of sales, why not ask for a "guarantee vs. percentage?"
In other words, ask them for a flat fee, up front, if their sales don't top a certain figure and a specified percent of the "overage" on sales that go over that figure.
For example, your "guarantee" would be $500. You get paid that much up front, no matter what their sales are. Then, if the sales go over, say, $2000 you would get 20% of anything that goes over $2000.
This ensures that you get paid something up front and they can't cook the books and screw you out of paying anything. Then, they get the benefit of paying a lower percentage on future sales. Hopefully, that will give them less incentive to tinker with their sales figures.
This is the way a lot of independent movies are booked in theaters. The theater pays a flat guarantee fee to get the print on-screen but, if the movie tanks and they don't sell any tickets, they aren't on the hook for more money than they have already paid. They only pay more if the movie makes more than a predetermined amount. Then, it is only a percentage of the gate, based on what both parties agree upon.
I don't know if it will work but, what the hell... It's worth a shot.
If they really are on the up-and-up, they'll negotiate on that. If they don't want to negotiate or, at least, make a counter offer, you'll know that, more likely than not, they are crooks that you probably don't want to do business with, anyway.