I assumed there would be a market for new aftermarket batteries but it hasn't developed at all. Theoretically a battery pack isn't the most complicated thing and electrical compatibility is a lot easier to achieve than for a ICE.
Notably the kid said the hardest thing about the swap was sourcing the replacement battery.
I really think given a few more years when millions of Model 3s/Ys reach their 2nd/3rd owners, we will see this become more routine.
Also people talk about pack replacements like they are a single monolith but many cars have packs composed of 10-20 modules. Some modules can even be repaired at the cell level. So costs can trend down over time.
2. You need to know a lot about batteries and even more about battery management system and then know how to integrate batteries + BMS into a car. Sure you might be able to reuse existing BMS for a car you are targeting but it will quickly become a bottle neck in your manufacturing if you do so.
3. Pairing of parts. Related to point 3. You might be able to build a battery pack, are you able to also force the car to take the battery pack? Car might outright refuse to talk to it, because it is not paired and OEM won't let you pair it, because "safety", but more like safety of their profits.
4. Shipping of Lithium batteries in general is huge PITA. Try to ship battery of any size with Fedex/DHL/UPS. They either outright refuse to or you will pay a lot of extra.