IMO, Condition and Mileage to me are more important than the Year, but the Year also has it's bearing of importance as well.
Go to
http://www.kbb.com/, put in all the pertinent information and it'll tell you what the going price is in your neighborhood.
Verify what other's are listing for the same model truck on Auto Trader.
I had a '93 Nissan Truck 4 cyl, AT, that I pulled a 5'X10' single axle trailer with,, and as long as the tailgate was left off,, it pulled pretty good,, so a 4 or 6 cyl,, should pull your 4 x 8 good.
I was surprised at the value KBB gave me for my '93. I verified the price on AutoTrader,, and was amazed that mine got 4K. Probly could have gotten more. The First Person that came and looked at it,, bought it. 20 year's old, with 176K miles,, and it sold, like it had Wing's Fast.
Good Luck