Prior to getting back into "powder burners" I collected and owned approx. 25 airguns, mostly older ones, as they were generally very well made.
I owned a Webley Senior (with both .177 & .22 barrels), a Junior and a Tempest. Only have the Junior now.
Your "find" is very nice indeed. It appears to be in excellent condition. I would agree that with the box, accessories and that it is an early piece in like new condition, it would be very desirable and would draw top dollar.
I agree with '2strklvr'............"keep it"!
The Seniors were very well designed, made and finished, the bluing is better than some new powder burning pistols!
The Senior has nice weight, a good trigger and at 10 yards they are great for working on sight picture and trigger release in the likes of your basement. I still have a few of my older air pistols and use them for that very purpose.