Obsolete on your site is current on somebody else's. The value is in identification, not in age.
Obsolete, superseded and discontinued are different things
Obsolete means the plant no longer uses it. Superseded means the manufacturer replaced it with a newer model that fits the same duty. Discontinued means it is no longer made at all. A single item can be all three, and each has a different market.
Superseded stock is the easiest to sell, because the equipment it fits is still running and the demand for spares continues. Discontinued stock is harder, but it is often the most valuable to the small number of people who still run that equipment.
Why plants keep it
Because throwing away a serviceable spare feels wrong, and because the person who would have to authorise the disposal has no incentive to do it. So it stays on the shelf, and the shelf stays full.
The result is a stores room where a significant proportion of the value is in items that will never be issued, and where the space they occupy is the real cost.
How to identify saleable obsolete stock
Part numbers are the key. A box with a legible manufacturer part number can be looked up, matched to a current or superseded equivalent and priced. A loose item with no marking cannot.
Failing that, the nameplate, the body marking and the pattern will usually get you most of the way, particularly on standard valve types.
What sells and what does not
Obsolete control valve actuators, positioners, relief valve parts and specialised trim sell well because they are hard to replace. Obsolete standard gate valves sell less well, because a new equivalent is cheap.
The pattern is consistent: the more specialised the item, the stronger the market for the obsolete version.
The practical approach
Do not try to value it yourself. Send the list with part numbers where they exist and photographs where they do not, and let the review sort the saleable from the marginal. That is faster and more accurate than an internal exercise.
And do not dispose of it as scrap before somebody looks. The difference between scrap value and equipment value on obsolete stock is often an order of magnitude.
Questions sellers ask
Is obsolete stock worth selling?
Often yes, and sometimes more than current stock, because the alternative supply is gone.
How do I know if something is superseded?
The manufacturer's cross-reference. If you have the part number, a buyer can usually establish the supersession.
Do you buy single obsolete items?
Yes, when the item is substantial or specialised enough to justify it.