I'm thoroughly impressed with Marketpress. I'm testing it for the first time today. It's fantastic to have alternatives to the pitiful WP-Ecommerce!
I did find a very small issue which I think you would want to know about.... I did a Paypal sandbox test and went right through the whole process. Awesome so far!
The only glitch I found was in the confirmation email. Because I have an apostrophe in my site name, this showed up in the email "from" as a character entity instead of the apostrophe. I was going to publish it here, but even with code tags it didn't work. :-) So let's just say it is the entity that I would expect for that symbol, but it's not user-friendly.
I am set to use the usual UTF-8, btw, and reading my email with Gmail. I would guess that a few other symbols might come through this way, so maybe a routine could be written to fix those on the fly.
Hope this helps. Very nice work!
PS: I was going to mention this on your forums, but it looks like only non-free users get access to that.