@Jeff,
Unfortunately, anyone and everyone can and does claim their theme is SEO'd, and there's no real way to tell if they're feeding your a load of BS or not, unless you actually know what good SEO looks like.

If anyone cares to know, here's what to look out for ...

If the theme you're buying has "SEO" listed in a bullet point list of features, then it's probably BS.

Maybe you'll give them the benefit of the doubt ... so send them an email and ask why they think their theme is so SEOwsome. If they don't have a convincing response, run ... far away. These are the people who are going to be presenting your content to the world. If they're amateurs, you will get amateur results. Run.

What's a convincing response? Well, that's up to you to decide. I'll tell you what ISN'T a convincing response ... telling me that SEO doesn't matter or that all WP themes are "basically the same". Again, run away from them.

I can tell you, in detail, why Genesis is coded properly for SEO. The same goes for Thesis. The same goes for Hybrid. If the author of the theme can't tell you EXACTLY what makes his theme SEO, run.