A long, long time ago, I managed my Teachers' Tibetan antique store in Greenwich Village for a number of years. During that time, I learned a lot about Tibetan art and antiques. Many people brought Tibetan art in to our shop to try to sell or to have appraised, and I sometimes consulted for the Tibetan wing at the Natural History Museum. This thangka does not look PRC-made to me. It looks decidedly Nepalese in terms of its geo-political provenance (not the ethnicity of its painter). My guess is that someone bought it back in the 70s, originally in Kathmandu, which would make sense for something showing up in a recent estate sale. The crudeness of the drawing and the quality of the paints are very reminiscent of what was coming out of Nepal during the heyday of the hippie era. It's possible that the thangka was painted in Dolpo or Lo Mantang. Frankly, I can't see the details of the iconography well enough to decide whether it is simply a bogus tourist knock-off or whether its from a very poor area in the southern Himalayas. Sometimes the old thangkas from the monasteries and temples in these areas show up on the market when those monasteries or temples commission new thangkas to spruce thing up. Just some observations based on past experience.
Pema Chophel པདྨ་ཆོས་འཕེལ