Actually, Big 5's sale prices are pretty darn good (IIRC, the aforementioned K-31 was something like $80). As mentioned, however, you really have to look around to find one that's in decent shape.
The problem with buying milsurps online is, assuming you don't have a C&R, the transfer and DROS fees alone can almost double the price of what you're buying. A local shop here charges $80, and if you add in shipping, that's a cool bill right there. If you're saving more than $100, or if you're looking for something particularly rare, or if you found something that's just cherry, then maybe it's worth it. IMHO, a C&R is pretty much a must-have if you're going to buy online.
Gunshops aren't guarantees, either. I've seen some real dogs at shops, and the asking prices were outrageous, too. I'd say your best bet (short of getting a C&R) is to hit up your local gun shows with cash in hand. Selection is usually better both in terms of condition and variety. Just know that a gunshow "discount" usually means +10% for some strange reason. Bring cash, make a fair offer, and be prepared to walk away.
Go ahead and take a peek when you're in a Big 5. Doesn't hurt to look, and who knows? You might get lucky. Go to their website, too. Sometimes they have 10% off coupons that are even good on items currently on sale. I bought my Marlin 1894c from them that way (originally $400, on sale for $330, down to $300 w/ coupon, ended up being $350 OTD
).
Good luck.