Selective Value Lists

I am making a simple asset management spreadsheet and would like to create drop down lists that show what parts are available for checking out.

Once the part has been checked out or assigned to an employee or ID# I would like that part to disappear from the drop down list.

This is to prevent single parts being assigned to multiple cells.

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OR if the drop down list idea can not be done than perhaps the value list in the drop down can change color or indicate a status whether in or out...?

ANy ideas or solutions appreciated.
Cheers
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