I spoke with someone who is willing to let me sell his Lego for a cut of the profit. about 100 minifigs and some boxed sets. Anyone done this before? What is a reasonable % for me to get for selling and shipping for him?

eBay charges 15%
I've been offered 20%
But it really it depends on your relationship with owner.
Two important things to consider. how much do you have to make for it to be worth your effort.
And how much is he expecting to receive.

So you can flat rate it, ' I think it'll take 2 hours of effort and 5 miles of gas. I want x dollars guaranteed.

Or he can say, 'I want $150 for it, anything else you can make off it is yours.'

I prefer the latter, more incentive to sell for more. But if his price is low enough, you can basically buy it from him and sell it at your own pace.

Personally, I wouldn't do this.  There's quite a bit of risk as a seller, including NSAD claims, paypal issues, etc.  You're the middle man in this, and there's just too much room for issues.

 What I've done for friends is offer to buy sets from them.  Then, I own the set and can dispose of them as I normally would.  

I have done this to help my nephew raise college money, and it worked great!!! We really like putting together sets plus I learned a lot about the used Lego market. I did not accept any money though, we did pick a few sets to keep for our collection for all the work though. The best part was he made $6,000 dollars which was enough for 1 year of college with scholarships also and one day hopefully he will make enough money to buy all the Lego he wants

if I was going to charge I would also offer to buy them though or do a percentage. It would have to be at least 30-50% as it takes a lot of time, research and knowledge to list a lot of sets. Plus your account is the one at risk and also have that tax issue to worry about for your profits.

This wouldn't be worth my time. At most, you are looking at a couple hundred dollars.