The answer to your question, as phrased, is "it depends."
Are you talking about just the axle or a complete axle assembly with bearings and brakes? What condition? Who is the potential purchaser? Someone doing a true restoration of their M100 trailer would place more value on an OEM axle than someone building an offroad camping trailer who would like to match the wheels on the trailer to a tow rig that doesn't require a 5 on 5 1/2" lug pattern. To the latter the OEM axle has zero value.