Yea that's not a Trichocereus pachanoi. It's definitely a Cereus peruvianus(common name - "Peruvian Apple Cactus").
Sorry. But I would still recommend buying it for sure...and then planting it outdoors where it gets plenty of light. Unless you live somewhere where it gets extremely cold during winter(In which case plant it in a large pot that you can bring indoors in colder weather). It grows large and in a columnar pattern....produces large beautiful flowers and large apple shaped fruit that looks like Dragon Fruit/Kiwi on the inside. Some say its even tastier than Dragon Fruit.
I actually just got like 5 cuttings of this cactus off of someone through the OfferUp website. They are HUGE cuttings that prob weigh ~40 lbs +. All I had to do was trade a couple agave suckers that I have all over my yard, an aloe sucker, and a few Dragon Fruit cuttings that I was going to cut off anyway. AMAZING DEAL!
So yea, buy the cactus if you aren't just looking for a high. If you are...you're looking at the wrong species anyway.... T. bridgesii is a much better/safer bet when it comes to that subject. But I recommend buying cacti to grow...not eat.
Good luck! ...and don't pay more than $20 if you can.
-cj-