I was thinking 4L other extensions to sell (.app, .io, .tech, world).
No, stick to .
com, you wont sell them, most companies have never heard of the new extensions. .IO is different as its not a new extension like world, app tech
You need to spend at least a few moths researching and reading through this forum from top to bottom or you will end up wasting money by buying crap names. Don't fall for the new domainer reg frenzy and go and start buying names, learn what makes a name valuable first, I can tell you havent done that yet as you are mentioning .world, .tech. .app etc
Biggest mistake new domainers make is hand-register names and park them and list them everywhere thinking they will just "sell"...if this was the case, dont you think that buyer would have registered the name himself for $10. This strategy has a greater chance of working if you spend a bit more (x,xxx or xx,xxx) and buy quality names.
First research and read this forum from top to bottom for a few months so you understand what makes a name valuable.
All this below only comes after you have read this forum for a while.
Check out DNJournal.com and Namebio.com to see what type of names sell
Check out expiring lists and see if there is any value in the dropping names.
Email existing owners of decent names and buy 1 or 2 names, you wont know what is decent until you have done your research
Make sure there are plenty of potential buyers before buying the name
Avoid hand-registering names that are available, they are usually available for a reason.
Emailing potential buyers asking if they are interested in the name
Don't get greedy when starting out and sell for a reasonable price
Reinvest that profit into another name and then repeat the process.
Don't mix letters and numbers
Avoid long names and dont buy 3 word names
Avoid trademark names!!! (Company names like Google, Amazon, IBM, Ford etc)
Stick to .COM domains
Avoid hyphens in domains
Hope that helps, good luck