License plates in Texas are governed by the Texas TRANSPORTATION CODE, TITLE 7. VEHICLES AND TRAFFIC, SUBTITLE A. CERTIFICATES OF TITLE AND REGISTRATION OF VEHICLES, CHAPTER 504 LICENSE PLATES
Texas code section 504.901. TRANSFER AND REMOVAL OF LICENSE PLATES (subsections (b) and (c)) provide that
(b) On the sale or transfer of a motor vehicle to a person who does not hold a general distinguishing number issued under Chapter 503, [i.e. not a dealer] the seller may remove each license plate issued for the motor vehicle. The license plates may be transferred to another vehicle titled in the seller's name if the seller obtains:
(1) the department's approval of an application to transfer the license plates; and
(2) a new registration insignia for the motor vehicle.
(c) A license plate removed from a motor vehicle that is not transferred to another motor vehicle must be disposed of in a manner specified by the department.
It seems that the seller is not required to remove the plates when selling the vehicle
Section 504.010 ISSUANCE AND PLACEMENT OF LICENSE PLATE. provides that:
(a) On payment of the prescribed fee, an applicant for motor vehicle registration shall be issued a license plate or set of plates
This seems to tie the plates to the registration, which means to the person registering them as well as to a particular vehicle.
Section 504.943. OPERATION OF VEHICLE WITHOUT LICENSE PLATE provides that:
(a) Except as provided by Subsection (b), a person commits an offense if the person operates on a public highway, during a registration period, a motor vehicle that does not display two license plates that:
(1) have been assigned by the department for the period; and
(2) comply with department rules regarding the placement of license plates.
Section. 504.944. OPERATION OF VEHICLE WITH WRONG LICENSE PLATE.provides that:
A person commits an offense if the person operates, or as the owner permits another to operate, on a public highway a motor vehicle that has attached to it a number plate or registration insignia issued for a different vehicle. An offense under this section is a misdemeanor punishable by a fine not to exceed $200.
It would seem that under 504.943 or 504.944 (and the related 504.945) the person who bought is committing a misdemeanor every time that the person drives it on public roads or streets. The seller could inform the buyer of this and request the buyer to get new plates, or report the buyer to the police as having improper plates, which would allow them to stop the buyer for having improper plates. Use of certified mail would preserve a record of the notification of the buyer. Note that notification is in no way required before reporting to the police, but might induce the buyer to get new plates on his or her own.
Secretly removing the buyer's license plate would probably not be theft, as it seems that the plates actually remain the property of the state, but would at the least involve trespass. It seems unwise at best.