Oh jeez. I've heard this before. The seller pays the commission. That's it. In many places in rentals the owner or property management pays the agents commission as well ( in most places this is the norm) you also can find " no fee " apts. Now your next theory will be that the rent is set higher to off set the fee!!
Your theory about more $$ for sale bigger commissions is laughable as well. Let's take Thailand for example you ask 4.2 million and offer is 4 million. Now to get this extra 200,000 the agent and seller are in cahoots to screw you over. Ok so 3% of 200,000 baht would mean the agent gets 6,000 baht more which is MORE than likely also split with the house/ broker!!! So depending on the agents commission structure they get maybe 3 or 4,000 baht more only!! Hmm would not the agent just convince the seller that 4 million is a great offer and they should take it? Then just move on to next sale. Many condos are also well priced or even underpriced. Maybe you should offer a higher price? Happens in the West a lot. There is a value in Real Estate. The commission is paid from the seller to the agent. It comes from an AGREED TO PRICE BY BOTH PARTIES.