For the condition you describe, the $550 is a fair deal if it's closer to 97% than 95%.
If the exterior of the gun is clean - no scratches in the slide flats or on the matte top surface, no dings or gouges in the metal, the the next step is to look in detail.
Ensure the gun is empty! Check it again.
Cock the hammer. With the safety off, push the hammer firmly with your thumb. It should not budge. If it falls, the sear is worn out.
With the hammer cocked,
apply the safety. With gun pointed in a safe direction, push the hammer again. Should not move. Pull the trigger. Hammer should not fall. Release safety - hammer should not fall. Pull trigger with safety off - hammer falls. This is proper operation. If the hammer moves when it shouldn't, fix the trigger & sear.
Make sure the gun is empty.
Cock the hammer, point gun in a safe direction.
Place your finger on the trigger with almost no pressure. Slowly & gradually increase the pressure until the hammer falls. You should need at least 4-4.5 lbs to trip the hammer which is a deliberate pressure. If it trips the hammer with a light touch, have a gunsmith inspect. (if possible, use a trigger scale to check the trigger pull.)
Insert an empty, quality magazine and pull the slide back. Slide should lock to the rear. If not, try another magazine and it should lock. If not, replace slide stop. With the magazine in place, retract the slide and let go. Slide remains locked open. Repeat, but depress the slide stop and the slide closes. Lock slide open, remove magazine, retract slide fully, slide should close when released.
Look at the barrel hood where it meets the slide when the gun is closed. Should be a tight fit. Look at the top of the barrel to see if there is a lot of wear (some wear is normal-flat spots are a bad sign.). Lock the slide back and look at the rear of the hood over the barrel - is it clean edged and straight or is it peened? Look at the mating surface on the slide-any damage?
Check the end of the flared bull barrel. Champions don't use a barrel bushing (at least mine doesn't). Can you feel any flattening on wear points? If so, close the slide (making sure the gun is empty of course) and try to move the barrel in all compass points. It should move very little if at all. The muzzle should be flush and centered;
Springfield Champion Muzzle view
If it passes all of the tests and looks as clean as the one below (pictured with a Galco Jackass shoulder rig), you'll be paying a reasonable price.
