The only experience I had with 2nd hand guitar shops was Cash Converters. During that time when I was selling my guitar, I was oblivious of SOFT, Guitar Connection or any other 2nd hand shops. And not being 18 at that time, I asked the help from an aunty to help me sell it. I was expecting to sell it at $150 but the shop gave me $80. I negotiated for $100 since I was giving them a gig bag with it. So I got $100, less than what I expected but better than nothing.
From my experience, Buy/Sell section of SOFT, where my price expectations are met. You got to give or take a times. My 2cents worth. =)