Around here an 880 in excellent used shape with bars and chains would fetch $1000.00+. The dealers would not however give much more than $500 to $600 for it on trade. They certainly would not even up on a 660. Most seem to feel that 880s are hard to sell. An 880 that has been hard used and looks it would go for $500.00 plus.
Private party sales are all over the chart. EBay has ruined the large Stihl market as people on Ebay seem to be willing to spend at least $350.00 on blown up, not complete, caught on fire, found in the river and assemble from garbage saws.
Ask the person what they want for it. What it is worth is up to you and/or your wife. There have been a couple floating around they central Ohio area lately. Definitely run it and flip it around on its sides. Also if it is warm when you get there I would walk away unless it was dirt cheap.