There are a lot of assumptions in that statement. You could be spot on but.....
Dont know what spring is in there in the first place. Could of been an m95 and shooting about 310/fps in the first place, that would work out to a 40fps loss which is easily possible with a shot spring.
I have seen a compressed spring drop 50fps. So it is possibly the spring.
I personally reckon its probably an air seal issue but wouldnt leap straight to a cracked box. Could be a shot o-ring on the nozzle coupled with a poorly fitted bucking that will drop fps massively.
There are lots of things that can cause an fps drop and most are pretty simple fixes.
Change, spring check seals, reassemble hop using a bit of ptfe. Takes 45 minutes and cost about £16. If that doesn't work then yes it might be a cracked box.
But without looking nobody knows.
Op should check all the basics to improve fps before sale and clean up the exterior.
If thats beyond him then sell as parts or lower the price maybe but lets not consign it to the boneyard on assumptions just yet.