Taxation is the same as mutual funds. Please see this post:
What you are referring to is market making. For market makers to make markets or provide liquidity, there has to be enough demand, which isn’t the case with a lot of ETFs. In some cases, there are other issues that market makers could face like funding constraints, regulatory restrictions, volatility etc. Having said that, in most of the popular ETFs, liquidity is fairly ok for small investors.