Hi, I am also very new here, however I would like to give Chellger my take on the BDD Fund, please correct me where I am wrong. I think my explanation below is oversimplified, but much better than saying B.SELL is shorting B.MINE:
While you get to essentially short B.MINE with B.SELL, I wouldn't call it shorting because their prices both derive from the bitcoin difficulty and not B.MINE. Also, when shorting a security you can potentially lose much more than what you spent on that security, whereas with B.SELL the maximum you can lose is the amount spent on the contract itself (i.e. the price at which you bought it). The nice thing that B.SELL provides is hedging your SHA256/BTC mining activities against difficulty increases.
Think of it this way, the sum of what B.SELL and B.MINE can payout (in total) is equal to the amount raised by the BDD fund from selling B.EXCH contracts. Buying the contracts at the correct prices is therefore your main concern as you can then get a bigger piece of the pie.
If you really want to delve in deep, consider learning more about derivative pricing models, as both B.MINE and B.SELL derive their value from mining difficulty.