For a Gen1 S2K the only solid alternative is a Gen2 S2K. Anything else lower budget doesn't compare (because 5 round mags). So just be cheaper than any Gen2. $550-$625 for good condition?
Good luck!
My original reply (I wrote this for Gen2 because I don't read good):
Assuming no upgrades, low round count, good condition ... ~$650 is the right price at this time. You might chance starting higher (<$750), but it's easy to find a new one for less than $800+tax in late 2023.
If you've done upgrades then ask more. For an S2K many upgrades are personal choices (expensive red dot mount for sure) and might be hard to get money back on from the same buyer. Itemise them or split out if you can.