I tested an install of MarketPlace and got the following errors. I've now removed the plugin to wait for a future release but wanted to report them for future development.
I created a product and added the product widget to my sidebar. When I clicked on the product in the sidebar I received a 404 error.
I then went back and created a category called "Services". Edited the product and placed it in the "Services" category. Added the product category widget to the sidebar.
When I click on the "Services" category in the sidebar I got one of my pages displayed inside the Marketpress layout. Without realizing it my site had a page called "Services" and the category link was now displaying that page. I tried changing the category name, but it didn't work I believe because the category slug was still "Services".
I then created a new category, moved the product to the new category and the category link worked fine. Clicking on the product still produced a 404 error.
At this point I abandoned testing on the plugin and went back to adding paypal buttons to my clients pages. I would love to use this plugin in the future so will continue to monitor its development.