Who is liable for damage to another vehicle when the owner sells the vehicle with the tag still on it and the purchaser does not register the vehicle.

I don't understand the last part of your question.  In terms of what needs to be done, who is the seller. Did he or she accept cash or check? Was the check or cash deposited? Was there documentation signed? Did the seller give over the title so the new owner can register.

Section 32-8-44

Transfer of ownership - Generally.

(a) If an owner transfers his interest in a vehicle, other than by the creation of a security interest, he shall, at the time of the delivery of the vehicle, execute an assignment and warranty of title to the transferee in the space provided therefor on the certificate or as the department prescribes, and cause the certificate and assignment to be mailed or delivered to the transferee or to the department.

(b) Except as provided in this section, the transferee shall, promptly after delivery to him of the vehicle, execute the application for a new certificate of title in the space provided therefor on the certificate or as the department prescribes, and cause the certificate and application to be mailed or delivered to a designated agent.

(c) Upon request of the owner or transferee, a lienholder in possession of the certificate of title shall, unless the transfer was a breach of his security agreement, deliver the certificate to the transferee. Upon receipt of the certificate the transferee shall make application to a designated agent for a new certificate. The delivery of the certificate does not affect the rights of the lienholder under his security agreement.

(d) If a security interest is reserved or created at the time of the transfer, the certificate of title shall be retained by or delivered to the person who becomes the lienholder and the parties shall comply with the provisions of Section 32-8-62.

(e) Except as provided in Section 32-8-45, and as between the parties, a transfer by an owner is not effective until the provisions of this section have been complied with.

(Acts 1973, No. 765, p. 1147, §13.)

Section 32-8-30

Certificate required for certain motor vehicles, mobile homes, and travel trailers; cancellation of title to certain mobile homes affixed to realty; reapplication for certificate when mobile home is detached; penalty.

(a) Except as provided in Section 32-8-31, every owner of a motor vehicle designated a 1975 year model, and all models subsequent thereto which is in this state and which is required to be registered under the motor vehicle laws of this state and for which no certificate of title has been issued by the department, shall make application to a designated agent as herein defined for a certificate of title to the vehicle.

(b) Except as provided in Section 32-8-31, every owner of a mobile home designated a 1990 year model, and all models subsequent thereto which is in this state and for which no certificate of title has been issued by the department, shall make application to a designated agent as herein defined for a certificate of title to the mobile home, or to each unit thereof if the mobile home consists of more than one unit.

(c) If a mobile home is affixed to a parcel of real property and the ownership of mobile home and real property is identical, the owner or owners may obtain from the Alabama Department of Revenue a cancellation of title to the mobile home by delivering to the department, the following:

(1) The certificate of title to the mobile home, or each separate certificate of title if the mobile home consists of more than one unit;

(2) A certified copy of the deed or other instruments of conveyance to the realty to which the mobile home has become affixed;

(3) An affidavit executed by all who have an ownership interest in the mobile home and the realty to which the mobile home has become affixed to the effect that the mobile home is affixed to the realty described in the deed;

(4) Lien release from lienholder as recorded on the face of the certificate of title.

If a mobile home for which the certificate of title has been cancelled as provided in this subsection is subsequently detached from the realty to which it became affixed, the owner or owners must reapply for a new certificate of title, provide an abstract of land title showing ownership of the mobile home and realty and any changes, if any, since the previous cancellation of the certificate of title, and pay the required fee specified in Section 32-8-6, for the mobile home, or if in more than one section, for each component unit.

(d) Except as provided in Section 32-8-31, every owner of a travel trailer designated a 1990 year model, and all models subsequent thereto which is in this state and which is required to be registered under the motor vehicle laws of this state and for which no certificate of title has been issued by the department, shall make application to a designated agent as herein defined for a certificate of title to the travel trailer.

(e) Any dealer, acting for himself or another, who sells, trades or otherwise transfers any vehicle required to be titled under this chapter who does not comply with the provisions of this chapter shall be guilty of a misdemeanor and upon conviction shall be fined in a sum not exceeding $500.00.

(Acts 1973, No. 765, p. 1147, §2; Acts 1989, No. 89-918, p. 1817, §2.)