There are fears for the future of Plas Tan y Bwlch ahead of a Snowdonia National Park Authority meeting about it this Wednesday.
One resident told the ‘Cambrian News’ “people are horrified at the thought of this historic building falling into private hands.”
Llinos Alun added: “A local group met to try to stop any sale until the public have been consulted on its future.
“They intend to attend the authority's meeting on Wednesday to bring attention to local feelings regarding this potential sale - a decision the authority seem to have made in a vacuum.
“A private sale for locals may mean they lose access to the gardens, woodland and Llyn Mair, enjoyed by many for generations.”
Llinos believes a Plas Tan y Bwlch staff member spotted the sale listing online.
She said: “They were aware of discussions regarding being put up for sale this autumn, but then it was just there for all to see. No staff briefing. Shameful.
“We’re desperate to spread the word about the land and footpaths being part of the deal; lots of people are unaware. “We are really hoping people come to show their objection on Wednesday’s authority meeting.”
Mabon ap Gwynfor MS and Liz Saville Roberts MP said: “Plas Tan y Bwlch is a centre of excellence in many ways. It has been the cradle of several national organisations over the years and has contributed extensively to an understanding of the environmental, historical, and cultural wealth of Wales.
“It is vitally important that Plas Tan y Bwlch and the land around the Plas remains under such ownership where the public can continue to benefit from the site, and we are in favour of the idea of safeguarding the centre as a cooperative resource.
“We urge the National Park Authority to work with local groups to develop Plas Tan y Bwlch into a viable centre and public resource where the range of old relics that have been given to the park over the years can also be displayed.”
An authority spokesperson said they won't comment on this until after Wednesday’s meeting when a press release will be released.