You won't pay taxes on sale until you go to the DMV to get it titled in your name. No need to visit the DMV until HE gets his title. Then he signs it over to you and you go register and pay TX taxes.

If you are buying it from him BEFORE he gets his title (taking the X5 with you) make sure you do a Bill of Sale with mileage, VIN, etc... get a copy of his DL, and then pay him.

Then you just wait for him to get the title and he can sign it over. After that you go to DMV, pay taxes, and apply for title.

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2005 X5 4.4i Build 04/05 Maintenance/Build Log
Nav, Pano, Sport (Purchased 06/14 w/ 109,000 miles) (Sold 8/15 w/121,000 miles)

2006 X5 4.8is Build 11/05 Maintenance/Build Log
Nav, DSP, Pano, Running Boards, OEM Tow Hitch, Cold Weather Pckg (Purchased 08/15 w/ 90,500 miles)

2010 X5 35d Build 02/10
Nav, HiFi, 6 DVD, Sports Pckg, Cold Weather Pckg, HUD, CAS, Running Boards, Leather Dash, PDC, Pano (Purchased 03/17 w/ 136,120 miles)