How annoying! People do it with cars as well. Big sign in the window "FOR SALE" no price, just a mobile number. It would put many off, even if they were interested. Likely the seller going to be be some chancer that thinks he will talk you into paying a high price.
If I really liked it, I would research how much it might be worth, then call to ask the price. Then, unless it's really cheap, make an offer that you are happy with "subject to seeing/testing/it being as nice as it looks in the photo's, etc." No point even going to look if the price is more than you want to pay. Then go and look being prepared to walk away if it's not right for you.
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