[consta]

That is truly an uncomfortable situation. Without any deeper knowledge about HamsterDB or your pro version, you might consider offering your service as SaaS and charging for that. Of course this won't work with all your pro features such as compression or encryption (or maybe it could work in some innovative way).

For instance you could charge for painless DB hosting, so users do not have to worry about complicated setups and can immediately use HamsterDB after signing in.

EDIT: But you can still go for the 30 day money back guarantee with the exposure of the pro version's source. If that fails you still can decide to switch to a SaaS model.

[cruppstahl]

Sorry if i was not precise enough - it's a C/C++ library that you link into your application. There's no SAAS interface. Otherwise the problem would be easy to solve...