Where did you see that a used 3d beaver imperforate sold for $50? My 2018 Unitrade Catalogue prices a used VG example at $75 CD and a used VF example at $300 CD. The example that you show in your post is well beyond VF and approaching Superb if sound. 3d beavers showing parts of two other stamps and with additional margins like this one are exceptionally scarce.
I think the Reserve is accurate. In an American Auction that stamp would sell easily for $500 USD plus premium. I have seen hammer prices as high as $750 USD for this quality.
Reserves are there to protect the Seller against Bid Rigging and Collusion, which is illegal but occurs in these stamp auctions in Canada. Buyers in collusion or other forms of corruption buy high quality stamps at low prices and turn around to sell them at the actual value. They generally make more than the original owner.