You'll most likely have to purchase a round-trip ticket for around $25 (depending on the day of the week)
I was there a few weeks ago and there were signs that said you could not buy a ticket to ride only from the top to the bottom.... You had to get a round-trip ticket. Not sure that has changed since then
When purchasing the tram ticket you are allowed to hold onto the ticket (no operator takes your physical ticket). The operator at the summit will put a line through your ticket when you are boarding to descend to prevent repeat trips up and down. So, yes, you can purchase a ticket, hike the mountain, and then ride down. As mentioned already, the cost would be for the round trip but otherwise would be fine since your ticket is as valid as any other rider from the top.
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