When translated from Spanish to English, "trailas en venta en NC" means "trailers for sale in NC." The letters NC are an acronym for a place name, likely North Carolina.
Although the phrase "en venta en NC" is correct Spanish, the noun "trailas" is a slang term commonly used in northern Mexico and the American Southwest. The term is an example of the linguistic phenomenon of code-switching, or the combining of two languages to formulate one idea. In this case, the speaker is using the English word trailer but utilizing Spanish spelling, grammar and pronunciation. Elsewhere in the Spanish speaking world, the word "remolque" refers to a trailer.