Look for the usual stuff.
1)What do you want in a fork- how much travel, how much adjustment (compresion damping, rebound damping adj , preload adj,, travel adj -maybe can have 120 or 100 mm of travel)do you want actual damping, or do you just want a cheapo sprung fork(you don't want just a sprung cheapo wal mart bike type fork, but just covering all the bases).
2) Brand specific-Rock shox, Manitou etc? Name brands are easier to find parts for.
4)Weight- all air usually 3.5 or less-metal spring about 4 lbs.
4) Defects-rust, corrosion, leaks, broken knobs adj , scored up fork sliders etc. Decent length steerer. What size 1" or 1 1/8" lots more selection on 1 1/8.
Make sure it has enough steerer length for your frame- VERY IMPORTANT- too long is no problem, too short is death-useless.
It is much easier to really check a fork if it is on a bike.
Too bad you don't live next door.I would sell you a Manitou Black elite120/100 with all sort of adj- 8.5" steerer for $85. Low use, low miles no real trail use.
Just don't be in a hurry to buy. You can bounce a bikeless for up and down to check to see if it still has damping-not as good as riding, but better than nothing.