Buying a trademarked domain name could get you into a lot of hot water. Best case scenario is for the trademark holder to force you into handing the domain over and call it a day. Worst case scenario is you get into a legal battle and end up spending an arm and a leg. This is why it might be a good idea to use the United States Patent and Trademark Office’s (USPTO) website to check for trademarks if you’re unsure about a domain name.
While buying up a ton of domains seems like a great way to make some extra money, the real world results show that it is very hard to make that process profitable. As with any industry, you will have those "golden moments" when someone you read about made it into a million dollar a year business, all while sitting in the comfort of his own home. That could be true, but he probably consumed a lot of alcohol and lost a lot of hair doing it.
Above you provided great information about buying and selling domain names. I have been to many blogs about the same and I have read the people make millions just by selling domain names. It's a process that one needs to learn with time and can give a good earning by reselling the domain name. The business is fruitful but one needs to read, understand the value of a domain name. Buying every domain name is a waste of money. One must see the future potential growth for that particular domain.
That said, my recommended budget for beginners would be $500+. Using this budget you could buy a bunch of high-potential $10 domain names, expired/dropped domain names or a mix of both. It’s very important to invest only what you can afford to lose and treat this as a side hustle till you get the momentum going. As you gain experience, industry expertise and some sales under your belt, you can then consider slowly growing your business into a full-time gig.