You couldsay: If someone knows what cell phone they want to buy, I will sell it to them. OR If someone knows what type of a cell phone they want to buy, I will gladly sell it to them. OR If someone knows what type of a cell phone he or she wants to buy, I will gladly sell it to them. The way these sentences are constructed produces the idea that the person selling it is either a salesman or some type of a dealer that can get any phone.

Technically, "they" is plural and "someone" is singular. So to be grammatically correct, you need the pronouns to match in number. An example of this is: If someone has an idea of what cell phone they want to buy, I will sell it to him or her. However, most native English speakers are not aware of this rule. If you are writing this sentence in a formal paper or something professional, use the above sentence or something similar. But when talking to native speakers, what you wrote is totally fine. Most English speakers would not even recognize anything wrong because in informal English, "they" is used to describe a person whose gender is unknown. If someone has an idea of what cell phone they want to buy, I will sell it to them. I hope this helped!

Hola Rak7 En inglés, lo más culto es decir "if someone has an idea of what cell phone he or she wants (que no se te olvide la 's') to buy, I will sell it to him or her". Obviamente no suena muy fluida esta frase, entonces la mayoría de hablantes de ingles usamos 'they' como pronombre singular para que no tengamos que decir 'he or she' cada vez que queremos hablar de alguien sin especificar el género. En la vida real probablemente oirás "If someone has an idea of what cell phone they want to buy, I will sell it to them." Nota que aunque 'they' refiere a una sola persona, todavía conjugamos el verbo siguiente como 3a persona plural. Muchos académicos quedan en contra del 'they' singular pero es bien aceptado en la sociedad además de ser lo más eficiente. Lo que has hecho aqui arriba usando sólo 'he' para referir a 'someone' es anticuado y solo se encuentra en los escritos de los años 70 y antes, entonces no recomiendo que lo uses...