Yikes!
Sure, it's LEGAL. It's LEGAL to ask you for a million dollars and then never do anything for you. It's LEGAL to ask you for your house--but you shouldn't give it to them.
I'd never pay anything upfront, and most agents only charge expenses against advances and royalites they receive when they have sold the book. And those expenses are accounted for on a line-item basis. "Adminstrative fees" are not legitimate.
Has the agent in question made sales to legitimate houses? I'd be surprised.
In any case, $750 is huge, and $750 upfront smacks of Scam City.
I recommend two steps. First, report this agent to Writer Beware and Preditors & Editors ASAP (though I have to say you are quite likely to find the agent's name listed at those sites already). Second, run like hell.
PS. Being listed in "Writer's Market," or any other list of agents, doesn't prove anything. Even some AAR members have engaged in some pretty damn questionable practices.