If it doesn't have a brand name of reasonable quality and I can't put my hands on the leather, then I wouldn't buy it. Definitely not for $300. However, there are a few good no-names out there, so if you're able to tell good leather then you may end up with a nice saddle!
If it does have a brand name, that'll help a lot in determining its value. The picture isn't really good enough to give much information, but it doesn't look like it's in bad condition. Ask about the brand, then run a google search for saddles of that brand, or post and ask the western experts on here
If no brand, my initial guess is definitely not worth $300 unless you are good at telling quality from crud when you see it.