Yes and if you don't want a sports band but a nice canvas strap you need to source it elsewhere because Apple doesn't offer one. Same for various other kinds of straps. Also, the 3rd party straps can be a lot cheaper than Apple ones. The strap that already came with the watch is free, you can sell it if you want or keep it as a backup or do something else with it. However, it is not a strap you can simply sell because you need to take into account that these can be purchased brand new from Apple and there are 3rd party straps that are cheaper. It is far more likely you are going to have to sell the strap for a lot less so keep that in mind when doing the math.
What you are describing here only works for collectors, not the general public. If the general public is going to buy something it has to work they way they want it and look pretty. If the general public finds this band to be ugly then the only audience you have left are the collectors which is a very small audience. In that case selling the band you cannot buy elsewhere is going to be a challenge. Just take a look at cars. There are certain times where a white car is very popular and there are times when no one wants to be seen with a white car.
Besides that, this also applies to those who already have an Apple Watch that isn't the Nike model and maybe, in the future, people with a Nike model looking for a replacement. If you don't have a watch then why would you buy a different model than the one you want (in this case, the Nike)?
I think you are not being very realistic about it which can lead to disappointment.
