2 identical guns, value difference if ?

Hard to answer, if you are being truthful you would put any accuracy issues in the advert I think, otherwise some wouldn't say anything and ask same price

As has been said value is hard to determine. You say same hole for one rifle at 45 yards with no reference for the 'inaccurate' one, is this say shooting at 5p piece group at 45 yards?
If they are secondhand rifles has one had more wear then the other, regulator differences etc.
How well have they been serviced and how often, does one have a dirty barrel? There are many things that could make a difference to the value other than accuracy, overall condition of the action, the stock etc.
The accuracy in some respects is almost irrelevant as many guns can outshoot the shooter and bear in mind that two rifles built on the same day by the same person could have marginal differences in performance due to the tolerances that manufacturers allow.
Finally how many people would be fortunate enough to have 2 identical rifles of the calibre of HW100 to compare back to back against each other?

The above are of course, just my thoughts.

I think I would be investigating why one wasn't as accurate and trying to sort it out before selling .

They have the same value, the next owner could be a better/worse shot than you, and of course the pellets used will be different also, so as long as they are both good, you can't differentiate price wise on guns like that, although if it were a comp winning BR gun you may get a bit more