Sell them your product and then get them to sign up to your list?!

You can (and should) automate it.

You don't need to ask them to sign up to a list if they're buying a product from you first. They're already your customers and you're entitled to be in contact with them.

Not unless you've told them very clearly before they pay that they're going to have to do this. Otherwise you're imposing a new "term of sale" on them after they've paid, aren't you? And your PayPal account may not last long if anyone complains. (If you take PayPal, that is: they really do close people's accounts down for this trick! You can see why, perhaps, if you think about it?).

You don't need to do this. I'd hate it, myself, and probably wouldn't buy anything from you ever again. If you require my email address to be submitted in a box before you deliver the product I've already paid for, then you tell me that upfront, please, before I pay, and give me the choice of whether or not I want to buy it on that basis!! (I'm not suggesting that I'd actually complain to PayPal, myself, if you don't, but I do know people who have lost their PayPal accounts over this, because someone did. It takes only one person to complain, once!).

After payment send the buyer to a page where they can sign up for updates for your product. People like to be made aware of updates so they'll generally sign up

Make sure to use single optin and more of the buyers will sign up, you could also talk about the benefits of joining your list. Don't worry about collecting the name, the email address on its own is enough.

When people receive things for nothing in return and they are presented with an option to provide something in return, they feel obligated to do so.

If you run the offer you should be collecting the emails automatically when they purchase. We make alot of our money on the back end by cross promoting other offers once they have bought. One thing you want to keep aware of, is to make sure you aren't just bombarding them with offers. You should be sending high value 2-3 emails and drop a soft offer in ...

Nurture the list and you can get tons of millage out of it. We have brought up our life time value of a customer from $80 per sale to around $130 by monetizing the back end.

What you are proposing is against PayPal's TOS. You should invest in a system that collects the buyers information automatically after they have purchased. That way you do not block their ability to receive the product they have just purchased.

Best regards,
Steve

I agree with automating your email list and keep in touch with your customers and potential clients without annoying them. Maybe set up a campaign once a week or so.