The collector value can only be assessed by a visual inspection & evaluation. Old Winchesters are very much like old classic automobiles... the graded condition and factory originality are the key factors in determining what they are truly worth.
Any one looking for .351 ammo can find it at Coins Stamps and Guns behind Elwood Epps ,North of Orillia Ont ,on hwy 11.
I heard a few rifles from 1917 and 1915 that sold for about $1,000
from 1907 through 1957 only 58,000+ were made, in fair condition it should fetch 500.00 for a first year production...
I myself own a Winchester model seven and I had it inspected by my local gun smith and he said it's worth around $2,225
The Winchester 1907 model was an autoloader made in .351 Winchester self loading cal. Civilian sporter models were made on up until 1937. The value for a good condition rifle with no stock damage and average wear would be about $400.( A version known as the Police model continued in production until 1957. They were very popular with Law enforcement and Prison authorities, around 59,000 were manufactured.)
Your Winchester model 1907 chambered in 351WSL(Winchester Self Loading)and was made in the last 3 years of production will bring between 175-450 dollars,for a rifle showing between 10%-80% of its original finish,and a good bore.These rifles were made from 1907-1957.
Try e-gunparts.com
Ammo for your .351 can be had at Old Western Scrounger. Go to www.ows-ammo.com and click on ammuntion.
50-500 USD or so
The Old West Scrounger usually has some.
Winchester made some that did. Remington also, IIRC.