Not quite sure where to post this, so I will try here ... anyone knows where are the NPCs who sell these houses?

The PES description says "There are two from whom you may purchase these, one located in Balmora, and one located near Ald Velothi." A quick peek at the mod shows that, yes, the 2 NPCs are placed in exterior Balmora and Ald Velothi cells. The Balmora one is a Breton called Andril Fornelin and the Ald Velothi a Dunmer called Adondinril Reynalori.