I posted my experience buying and building up a BXT135 in the respective thread. I bought it at the BXT store on Aliexpress. It’s a superb frameset if you can get it for a good price IMO.
I purchased it without seatpost and handlebar for 460EUR including shipping via DPD which includes customs and taxes. If you don’t want to worry about VAT, I recommend that. For that kind of money that frame is a steal. Now 610 is a different story.