Re:
Given the 300LX belongs to the early 90s (around 91-92), even if NOS it's very likely the trigger shifters won't "click" any more. Take that into account on the price. Sure you can try to fix them with WD-40, but NOS doesn't mean fully functioning here.
Since the shifters are attached to the brake levers, if they don't work that should lower the price of the combo.
Brakes at that lever were mostly plastic, so very low value even if NOS.
Crankset is pretty good, as they have replaceable chainrings, but they are steel (yes, heavier, but way longer lasting than its Deore counterparts). Just check beforehand if they are the Biopace type and if yes, if you are happy with that.
RD and FD are quite decent, and on those years the FD could still be opened with a screw driver, so you don't need to split the chain.
New 7 speed cassettes can be had for 10 EUR online.
So all around, I would offer 30 EUR or whatever equivalent in Queen's money, mostly due to the steel crankset.