Getting them across the border is probably the main concern. They will have to be quarantined and proven free of Brucellosis, Tuberculosis, no ticks, no lice, etc. I've heard of whole potloads of cattle being held up because one cow had a dead tick still attached. They're just doing a good job to protect US cattle, cattlemen, and beef consumers. Once across the border, you could probably sell them just about anywhere you wanted to. If they were going to ship to another state, you would need whatever "Health Certificates" may be necessary. Probably any South Texas cattle veterinarian would be able to help you with this. If you have a buyer already located in the US, he/she can help arrange all of this as well as transportation.