I notice you are selling file downloads, so it would not be possible to have product-specific thank-you pages, because that would bypass the standard thank-you page we generate for you where we provide the buyer's download link.
If you have product-specific information to provide to the buyer after payment, you can customize the thank-you pages we generate for you. Whatever you provide in the Product Thank-you Page HTML field of each product's settings would be inserted after the name and link for that product when we generate the thank-you page.
You can also put common information in Seller Admin > Edit Preferences > Common Thank-you Page HTML. Whatever you provide there would be inserted at the end of every thank-you page we generate regardless of which item(s) the buyer purchased.
If you prefer to redirect all buyers to a thank-you page on your site and still have our download links displayed there, it's a bit tricky to set up, but this help page explains the basic approach:
http://www.e-junkie.com/ej/tips.redirection.download-link.htm