I used to think that Interior Design was an easy job, not worth paying someone for until I gut renovated my house and suddenly my wife and I had to decide on 1000 little details that I didn't even know existed, all interdependent on each other. Everything from cabinet height to which 3 shades of white we needed on walls and baseboard and ceiling (who knew there were so many variations of white?) Now we build in a design fee on higher end properties, because it's hard and a skilled designer will actually save us time and money and aggravation.
I used to think that wedding planning was a total BS job until I ran my own wedding and spent I don't know how many hundred hours making phone calls to vendors, researching venues, lining up bands, making hotel arrangements, figuring out table arrangements, figuring out how many staff to hire, picking out silverware, and trying to figure out where to put the tables and chairs. Now whenever I am talking to a young couple planning a wedding I advise them to hire someone to help them. It saves them time and aggravation, and they will likely break even on money saved by using a pro who knows what corners can be cut vs the pro's fee.
Likewise, I used to think being a real estate agent was an easy job and agent commissions were a racket. So I got my license to make some easy money on the side and save a few bucks on my own deals. Then I spent my first few years going to dozens of open houses with unqualified buyers who never closed a deal, figuring out which inspectors and photographers and structural engineers and lenders and home stagers were worth a d*&! and building relationships with them, learning that most of being an agent is actually being a shrink/couple's therapist, and generally not making very much money at all before I figured it out. Now I turn down 75% of the people who want to use me as an agent and only choose clients that I don't think will waste my time. If someone tells me they want a discount, I tell them to hire a discount agent. My time and experience and relationships are valuable, and if I am able to help close a deal in only 12-15 hours of work, it is only because I have a decade of experience (generally unpaid) that made that possible.
If you are a good, solid customer who is easy to work with and respects the agent and their advice and brings them money that is actually easy for them to earn, it's fine to ask for a volume discount. But be honest with yourself. If you are a demanding client and you don't respect the professional that you have hired to market your property for you and you are nickel and diming them and pricing too high and not letting them stage the property to save a few bucks and generally making their lives harder, be prepared to have them fire you as a client.
And if you are at the point where you are selling so many properties that the 2-3% it costs you on the seller's agent side is a significant amount of money, go get licensed yourself and sell your own properties and stop paying anything (except for E&O insurance, staging and photography costs, MLS fees, gas, mileage, and all the other costs the agent is covering in that commission, of course). Many successful flippers and home builders do this. But if that sounds like a lot of work, well, that's why you pay someone else to do it for you.