To answer your two questions:
1) An SSL Seal in no way affects the purchase/sale on a technical level. This is the case for any site including Amazon.com The seal is window dressing. In fact ssl itself is not technically required to process an order. It is just a method of providing a more secure data transport between the end user's browser and the server that is processing a website.
2) On the psychological level however a seal is definitely a positive factor both on a conscious (for some) and unconscious level. Studies have shown and companies like Hackersafe will provide you statistics like "15% increase in sales" when you display their Hacker Safe logo on your site. The idea is that your site will close more sales because of a higher confidence level. Realistically the threat of hacking at time of sale for 99.999% of transactions on most sites is almost 0. Most hacking occurs in databases not on the transaction level and with the exception of things like keystroke logging it would be much less profitable for a hacker to spend their time on individual transactions as opposed to hacking a database with 10s of thousands of customer records. But banks and "security" companies play on the fears of the general public despite the fact that Internet commerce is much more secure (and pretty much always has been) than say giving your credit card to a waiter or gas station attendant.
And if you ever want to see the full weight of American law enforcement come to bear with terrifying efficiency, just try stealing credit card data. Terrorists, Serial killers, rapists, kidnappers, child molesters are a distant priority in comparison. Sadly that is not an exaggeration.
For anyone who either buys online or has an eCommerce shopping site, will the absence of an SSL Certificate Seal affect you from making a purchase or a sale even if it is going though Amazon's check out system? Conversely, will the appearance of an SSL seal on a site, encourage you to make a purchase?
