An associate at work offered her Walther PPK for sale.  It bites her hand when she shoots it so she purchased a 40 cal Glock.  
I was interested and sight unseen offered $350.00.  She said "I trust you."  
From there we went on line together to GunBrokers.com for comparables.  Hers was a year and 2 months new, purchased from Bass Pro, shot 25 or less times, Stainless, with one mag.  On the Net there were a lot of PPK's in the $600's, $500's, and $400's.  One was for $350.00 and claimed NIB, SST, one mag - just like hers.
We cemented the deal for $350.00.  I wrote her a check.  She took it.  I wrote out a Bill of Sale and made two copies.  She and I signed it.
Now, looking through her box, taking out the foam inserts, we found the original Receipt from Bass Pro...over $620 incl. tax new.
Right then one part of me was feeling like I took her to the cleaners - the other was saying what a savvy deal I just made.  We departed "friends" but I have this nagging in my gut feeling that I should offer and ante up another (say) $100 to $150.  I am "sleeping on it".
There are other SST PPK's on the Net for $395.00 to $495.  I don't feel so badly right now...but that will pass!