The first rifle I ever bought was a M77 tang safety in 25-06. Still have it, tho it now wears a Shilen stainless barrel. Ruger made a lot of great tang safety variants over the years. I've owned a few. Two I woulda, coulda, shoulda bought were the .250 Savage and the 7X57. I used to refer to the "recoil pad" as Ruger's Red Rubber Shoulder Bruiser... Had one in .338 Win. Mag. for a brief period... LOL! I've owned several and always loved the tang safety because I grew up shooting a side by side shotgun.
But, I eventually moved on to Winchester M-70s.
I really don't know the value of your 7X57. There are still guys who appreciate a well made, vintage walnut and blued steel bolt gun... and have an appreciation of classic calibers. Were it mine, considering the nice condition, and considering you'll take a 20% or so hit on the commission, I'd start at $1000. Yeah. I know some folks would laugh. But, if your gun shop has good foot traffic, and people can put their hands on a nice, scarce M-77 like yours and handle it, I'd think it should sell.
Anyway, best of luck.
MB