I am interented in Jms Multisite but would like some info before purchasing.

Can each of my Slave sites have Virtuemart where each individual site owner can add their own products without having access to any of the other Slave sites products?

Can the products from all Slave sites be shown on the Master site?

I can only use one payment gateway in South Africa called "Payfast" which is available in Virtuemart, will I have a problem with this once I've installed Jms Multisite?

Yes you can have specific VM for each slave site. So in this case, the master or any other website will NOT see any products of the other websites.

If you want that a website see the product from other websites, you need to share the content of the extension. In this case both websites will see the same content.

Concerning the "Payfast" payment method, I suppose this is an additional extension that you can add to VM.
If this is under VM 1.1 then this mean that the configuration of the payment method is shared between all the websites.
If this is under VM 2.x then VM now save the configuration in the DB and therefore is specific to reach website.
So this depends on the VM version and this also depends on the payment method where it saves its configuration.

Thank you very much for the info! I think I will give it a go