Sparks Sale - Sept 2020 Expensive 3-D Beaver

I was told that these 3d stamps sold for $50. Take a look at this one in the current Sparks Sale - Lot 21 opening at $675 on Sept 22.

How come this one has an estimate of $750?

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.

The lots like #14 or 15 would be more likely to sell in that range if it did not have the numbered ring cancel and had a smudged or indistinct cancel.

Scott/Unitrade 4d thin paper variety with jumbo margins.
(Unless mistaken, I think I can see horizontal laid lines on right margin closeup. If so, then it would make it Scott/Unitrade 1.)

If one looks at the auction listing, there is clearly a reserve placed on the lot. I assume it's the consignor's "I don't want to sell it but..." price.

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.

It appears that lot didn't meet the reserve in the auction and didn't sell, from looking at the realizations. There were a number of early Canada in the same situation.