Probably scrap value but if it is in a reasonable structural condition with no dents or fractures or fatigue cracking or galvanic corrosion pitting and wall thickness wastage anywhere including under the stainless fittings - need to remove the stainless fittings to confirm - then someone could be interested. When you look up the sail luff track is it straight laterally? Obviously fore and aft bend in the mast is expected but not laterally.
Might be worth posting it on gumtree with good photos and description of operational and maintenance life, any repairs done, and if the standing and running rigging is included. If so the age and condition of the standing rigging. Include length and cross sectional measurements, wall thickness, any internal sleeves at the lower panel, who the oem was, what cat design it came off so prospective buyers can judge if it might suit them.
Is a 30 year old aluminum mast off a 40ft cat of any value other than scrap value?
When I was building my boat and chasing a mast there were very few about.
Get all the dimensions and plenty of pics then advertise everywhere - Facebook Marketplace, also a group called Australian Secondhand Sailing Stuff; eBay; Gumtree etc.
Do a search to see what similar are selling for.