The "Don't like it, don't sell here" is true, you're right! but for me, that's my issue with fiverr - They can set their own rules and price, but is it the right thing to do? it's my belief that that saying is a good-ole-classic monopolistic statement that is immensely unjust. It's the same level of argument when people say "you should do what I say because I'm your manager and I said so" it's true they have authority, but it provides no productive valuable discussion. unfortunately that's just how capitalism works i guess. (unless like you said, a competitor rises up in the market and competes with fiverr, so fiverr has to change their way).
Do you remember the public uproar against uber when they exploited their workers by classifying them as contractors even though they legally should fall under the catagory of employees? thus removing the workers from rights such as sick leave, insurance, annual leave, etc. Uber can use the "Don't like it, don't sell here" statement, but Uber then were scrutinised by the Fair Labor act and fortunately they are now changing their policy to be more fair towards their workers.
With that in mind, can you imagine the uproar the public will have if uber starts taking a % on their workers' tips? Fiverr is lucky that they aren't a big enough company to catch the general public's eye and lawmakers, but I hope that they will soon.
You're 100% right that tipping isn't a norm in a professional exchange, as they are normally bound by contracts and tipping is more common in hopitality etc.. But I'm not trying to discuss about whether tipping is professional or not, but when a buyer decides to tip, seller should keep all of it - it's the right thing to do. If a seller doesn't believe tipping is professional, they can choose to not accept tips.
From what I gather . I think it we just have a different fundamental belief in this matter. I'm trying to start a discussion with average people like us, so that sellers like you, me and everyone else here gets treated fairly and are protected against corporate exploitation. and it goes beyond fiverr taking a % of tips, but when and if they decide to create new rules that is unfair to sellers in the future.