About that: are there recent reports of people, who haven't received their key after purchasing it via Paypal? As until now it seems that process was automated.
For me the very few terms of the PRO key on the Kisslicer website are clear:
In general, there is no return policy. Once you have the PRO Key we cannot revoke your license, as KISSlicer does not (and will never) 'phone home'.
KISSlicer PRO License
MAY be used for commercial applications
MAY be installed on multiple personal computers
MAY be backed up
The PRO Key MUST NOT be shared
MUST NOT be reverse engineered
So, no matter how you look at it: Kisslicer's PRO keys are not allowed to be shared: actually: your email (Paypall) adress is in the key. Kisslicer is owned by Jonathan / his family. That it isn't developing anymore is not a reason to take the right in your own hands and sell / share the key.
Under assumption the PRO key is still automatically sent after a Paypal payment I suggest that's the way to do.
When's actually not sent anymore I suppose you can get a refund via Paypal.
But, for me, when it's not possible to get a PRO key anymore (or for PRO users who have lost their key), I can imagine that we help each other with sharing a key, under the condition we pay Jonathan when it's possible again. But, to keep track of the keys, a PRO user can't share more than one key, so it's clear that when it got distributed more than agreed, you know who's responsible.
I know this is a tricky one: formally you can't do this, but OTH we are in a landed in a unexpected situation.
I can imagine as well, that when Jonathan returns and there will be a development of the slicer, he is able as well to change the necessary keys for the next release, and renewing them for known users, making the illegal keys unusable for new releases.
Bart