There are quite a few companies that offer this business model out there. They generally acquire points at a cheaper price in some way and offer to sell you cheaper than RRP premium tickets via award redemptions. Now I am not familiar with businessclassclub in particular or nor have I had experience dealing with these businesses personally.
Overall, it depends on your risk appetite. If all goes well, it may be an easy way to fly premium.
However, as with most things in life, there is always the flip side of the coin. I have heard of a handful of horror stories (not from businessclassclub in particular). The person’s ticket was cancelled at the last minute and all other related travel bookings like paid tours, accommodation, etc. were at jeopardy because of that.
Some frequent flyer program perform audits (e.g. Virgin Velocity has been rather strict) to ensure there is no trading of frequent flyer points. Repercussions I have heard of includes accounts being frozen, etc.
If you are like me, rather risk averse, buying points/miles during promotion times may work out to be of similar value and allowing you to fly premium at non-premium costs.
The above is just my 2c. There might be others out there that have great experience going via businessclassclub or other similar business.