That really depends on what all you need.
Are the seminar attendees going to need a file (a ticket to print, event schedule, speakers list, etc.)? If so, you'll want to use the Digital Downloads product type.
After the seminar, will attendees have access to special content on your site (transcripts from talks, posts by speakers, etc.)? In that case, you'd want to use the Membership product type (which also has the option to add downloadable files for the attendees).
If you don't need any of those things, you might just use the Simple Products product type. It is included in your Exchange core add-ons.
Let us know if you have any additional questions.