Not like that.
Private purchases are much tougher to get away with at a level like that.
Sometimes you can get a bigger commission, but you really can't represent a 500 thousand dollar horse to be a million dollar horse. Its pretty obvious what a horse is in the ballpark of after it runs.
What does sometimes happen, which is legal, is that an agent who has a lot of money(more than me, I don't have a half mill under the mattress) will buy one privately and then sell it to client at a markup. Thats not illegal, ist called arbitrage, or capitalism.
There is no standard commission that someone has to pay you. You can agree on 10%, 5%, 20%, etc. As long as you only take the commission from one side then you are ok. If you take say, 10% from the seller, and then bill the buyer 5% without disclosing the 10% you took from the other side, you are on thin ice.
Syndicates do this all time. One of those partnerships may buy a prospect for 350 grand and then sell pieces of it at a value of 500 grand.
The auction thing is much different. In cases that Richi describes, its outright fraud and price fixing.