$0.01 Payments
I've had a couple of people try to scam me recently by sending payments of $0.01 to my Paypal account, hoping that it will still trigger my download script for them.
I don't use encrypted Paypal buttons, downloads are sent by email, and my script emails me a warning if the amount of money received does not match up with the price of the product.
For buyers:
There's really no need for it, most sellers (myself included) will be willing to negotiate a discount on their products, if you don't think you can justify paying the full price. Some may let you have the product for free, in exchange for your services in other areas (can you design? code html? write articles?).
For sellers:
This is why it's important to have a good product delivery system. Like burt, I recommend Secure Delivery for this, as you can't beat it for ease of use - you can be set up and selling a product (at no cost to you, for the first one) in minutes.
Most of my products now use SD. There are one or two that don't, and these use a script coded by myself, as they need to perform other tasks in addition to sending out a product.
At the very least, secure your "buy" buttons, make it a challenge for the product thieves 
Comment by Jonathon — April 4, 2008 @ 6:06 pm
Hi Oli,
I'm using the RAP system to sell and deliver products and even that has had to be updated due to the $.01 paypal pirates! I totally agree that you've got to protect your downloads. It's not a good feeling to visit the web 2.0 sites and find that your download link has been saved there for the whole world to see!
-Jon
Comment by Chance — April 4, 2008 @ 9:16 pm
Secure Delivery does not use encrypted checkout urls for Paypal- we instead check to make sure the amount paid is equal or greater than the product price set up in our system. If the amount is too low then the delivery of the product is denied.
Buyers are free manipulate the URL to pay you too much or too little, but they won't get the product if they pay not enough
Comment by SarahG — April 5, 2008 @ 11:00 am
I use the same method as Chance when using PayPal IPN. That plus checking that the correct email address has been paid and the TXN ID hasn't been used previously. I can only assume people are trying this scam because it works on some sites. Makes you wonder how many insecure PayPal scripts there are out there.
Comment by Stu @ PMPR — April 6, 2008 @ 3:10 am
I'm amazed - I would never have even thought of trying to pay less than the ticket price for something online.
Mind you, I'd never have thought to send an email to an online vendor to try to get a discount either, although I've read here Gary talking about prospective customers doing it.
Wierd!
Comment by burt — April 6, 2008 @ 12:02 pm
It's usually confined to the "make money online" wannabees and hanger-ons. These "buyers" have the technical ability to understand how to change a paypal payment amount - most normal buyers don't.
"I recently ordered your {removed}. For some reason it says "email address logged possible fraud" but no download link.. i have no idea what this means, but payment has been sent. I would really like to download the product I purchased if you could send me the link that would greatly appreciated."
My reply:
"This is because on October 16th you attempted to defraud me by trying to purchase the {removed} for 5 cents. I will refund your {full price} payment in a few moments."
His (abridged) reply:
"There are alot of un-ethical business dealings taking place on the internet I have apparently fallen victim to one. If i was dis-honest I would not be having this correspondence, because dishonest people have nothing to say. I was just the unfortunate one caught in the middle of one of the many internet scams."
Whenever my script catches an underpaid buyer, the email address gets added to a list of "bad people". Future payments from them are accepted, but download links are not available to them (unless I manually authorise it).
Bunch of fraudulent wankers - would be the best way to describe these people.