^ You're not buying to/from BX. BX is only a trade platform. You're buying/selling off other people, and for each trade BX take a 0.25% cut.
Again, you're comparing prices with other exchanges when trying to talk about buying and selling on only BX.
I bought ETH from a trader on BX yesterday at 44,500.
Paid 44,500thb, minus 0.25% fee.
It's now listed as 51,850.
People are bidding 51,850, people are asking 51,900. I can sell right now to the bidders at 51,850, and minus the 0.25% fee take the 6,350thb that it's gone up and transfer that out to my Thai bank account.
There's no 10%, 15% or 20% lost anywhere along the way.
Buy and sell on the same exchange, and it doesn't matter a jot what people on other exchanges are buying/selling at.