Unfortunately I've had the same experience. These days before I buy anything I check the reputation, user comments and then email the developers asking them to guarantee compatibility (or commit to fixing it if claimed compatibility doesn't work).
The developers that worry about their reputation will either refund or "swap" the paid money for some of their other modules which actually work as advertised.
It's unfortunate this happens though and I'm not sure why really.
Boonex has started some discussion to try and resolve some of this, for both vendors and clients, but the generally accepted process these days is for software vendors to offer time limited trials, if the client is happy with it and it works, then buy, otherwise delete it. The problem with this model in Dolphin is once the client has downloaded the software, there's nothing to stop them keeping it.
From other discussions in forums, it seems the only way control of the installed module can happen is through "licensing servers", which developers are taking the time to implement instead of Boonex.
When I look around other social networks it seems to me a bit better organised for the vendor, the third party developer and the client.
Unfortunately all I can do is empathise and share with you any gotcha's and experiences to avoiding problems in the future.