The exemptions discount property tax by "exempting" a certain amount of a home's assessed value from taxation. ...

Again their incentive is that you are offering them a discount overall.

You have to buy at a super duper discount AND get repairs done with a good contractor or do yourself to have over 20% equity in a house almost immediately.With the tax value at 200k you probably have a retail price of about 215, maybe 220k.

While some may discount a few hundred bucks a unit.  

Instead the lender sells the loan, often at a deep discount ($30k for a loan with $100k balance for example) and writes off the rest.This is where a note investor comes in, they buy the loan for $30k, then go to the homeowner and restructure.

Could I take a discount from purchase price and not receive any commission? 

You are not getting a discount for that.

, but that if I don't use a broker the seller won't discount the price of the apartment (bc they don't want any other buyers in the building to think I got a better price).

Can I get a discount if I help you?""

But as @Chris Hopper said there are condos which can work even with HOA fees if they are bought discounted.