So pretty much when you force users in MarketPress to be logged in to complete and order they are prompted to either login or register. You mention requiring users to sign up may cause a decrease in conversion, however there are some things you could do to help.
When a user is forced to login to proceed with an order and they choose to register, they abandon the cart while registering, and upon completing their registration they are not redirected back to the checkout process. Simply create a custom registration where they are directed back to the cart to resume the checkout process.
Maybe allow them to even select a password to skip the email verification process? If worried about spam add CAPTCHA? The reason I require registration is because I've modified the shipping field to retrieve the email of the registered user and make the field read-only + hidden. I've also set the "Special Instructions" to put the username of user read only + hidden. This allows me to offer coverage of the purchased product to that user account only.
I know I have been making a lot of suggestions lately :stuck_out_tongue: Hopefully not too much!