I would suggest you put it on ebay.
(We don't really have a "for sale" forum here. Also make sure you have permission to sell it, especially if it is registered to your employer, else there could be surprises if the new owner registers it again.)
But how many people are lookin for something like that on ebay though?
Got a much better audience for that sort of thing here.... more people here know what it is and its value than ebay.
But yeah... make sure it's cleared with your boss, management, accounting, instrument tracking... surprises are bad... :uhoh
Searching ebay for "National Instruments" nets you 1600+ items. Many have bids.
Most are used NI products (but there are other "national" instruments, such as a banjo )
There is currently a PCI-6115 for sale for $1300 starting bid.
And not purchased by an NI Alliance member company, under the "internal use only" clause.