You only really need to install SmartDD on one domain, and you can then generate button code for the sales pages on other domains if that's all you need.

You just create your product and items as you would for eBay, and then go to the PayPal Button Code Generator for that item.  Then get your code and paste it onto one of your sales pages.  SmartDD will then handle the purchase (via PayPal) and delivery when someone clicks on it, and it will do so through the site you generated the button code on.

If you want to install SmartDD on other domains, remember that only one of them will be able to handle eBay purchases as you have to set that up as "static IPN" within PayPal.  PayPal's "dynamic IPN" will work with all your other sites.

There are two ways of setting up other domains.  If you have for example, a "reseller hosting" package (WHM - Web Host Manager)with your web host, then you can just create new domains up to your resource limits (usually).

If you just have a basic cPanel account then you can use "add-on domains" to create other domains off your main domain which works just like sub-domains.  i.e. the new add-on would be under your main domain as a sub-directory, and would look like this:-

/public_html/add-on-domain_01/
/public_html/add-on-domain_02/
/public_html/add-on-domain_03/

When you install on any of your add-on domains, and you're using the SmartDD Windows installer, SmartDD will (should) work out the installation location for you.  If you're doing it manually, the location of each installation as an add-on domain would be underneath your main domain's web root as a sub-directory.  Again, just like sub-domains.

/public_html/add-on-domain_01/SmartDD/
/public_html/add-on-domain_02/SmartDD/
/public_html/add-on-domain_03/SmartDD/

Look at this page to make sure you generate a correct licence for each add-on domain:-

http://www.smartdd.com/help/digital-delivery/tips-and-troubleshooting/

and read the bit about "The Licence Won't Work".

As for your databases you can create totally separate databases if you want to.  Or you can even use a single database for all your installations, as long as:-

a) they're on the same server (which they probably are if you're using add-on domains)

OR

b) you've enabled remote access to them if they're on different servers.

All you need to do is use a "table prefix" when going through the database setup stage at the very end of the installation.  So for each domain, you could use 3 or 4 characters to remind you which domain they're for.

For example, if you had a domain called exoticplantsecrets.com you could use a table prefix of eps_ and for classicaldrawingsecrets.com, you could use cds_ as the prefix.