$850 seems to be about the average price for a used Ruger. It really depends on the model and how old the gun is.
The newer Rugers have Briley choke tubes that are an improvement over the original choke tubes supplied by Ruger. The Briley tubes are longer ( about 2 1/2 in in length if I remember correctly). If the tubes are short this is an older gun . Ruger has steadily improved the guns through the years and the newer guns have better barrels and better balance.
Check out the stock, they tend to crack near the wrist (the part of the stock directly behind the receiver) if the stock bolt is tightened too much.
You didn't mention the barrel lengths and this will have a bearing on the resale price, longer barrels usually sell for higher prices. With 30" barrels going for more than 26" barrels . Generally longer barrels are more appropriate for Sporting Clays.
Does the gun have a rib between the barels. The newer sporting clays models do not have this rib but some older guns may have had the rib removed. Generally no rib is more desirable.
Check also the ventilated rib on top of the barrel. Is it loose? This will not affect the handling or accuracy of the gun but can be annoying if it rattles. Ruger does not solder the entire rib to the barrel, only at the front and back, relying on dovetail slots to hold the rib on. About 1 out of every 3 guns has a distinct rattle when you shoot or even when you open the gun. Mostly annoying but a problem for some people.
To check the tightness of the action, place a sheet of paper between the breech face and the barrels and close the gun. You should not be able to remove the paper when the breech is closed. If you can, the action is worn and may need to be repaired.
Look for gouge marks where the barrels meet the receiver. This will tell you if the last owner has mishandled the gun. If he has abused the gun when assembling the barrels to the receiver, you will notice scratches or gouges on the top of the sidewalls of the action near the front edge.
Check the chambers, do you see rust. this is a sure sign that the bores have not been maintained properly. Look for pitting and discolored areas in the tubes when you hold them up to light.
Lastly look for dings and scratches on the barrels. Check near the muzzle to see if the bluing is rubbed off or if the edges are rough, a sure sign that the gun has been mishandled.
Geoff Ross