Texas state laws consider the sale of a vehicle by a lease company at lease-end to be a separate sale, although a special one, and the lessee is not charged sales tax again if the correct amount of tax was paid originally at the beginning of the lease.
What this means is, if the current lessee has paid Sales Taxes for the full purchase price of the car or he has rolled them into the lease, then you are not responsible for paying the sales tax again at the end of the lease if you want to use the buyout option.