I need to buy a specific thing to complete a thing I have...

mossygreen thought this was worth mentioning said

…and someone is selling the exact thing I need on mercari. It is an unusual item and this is the first one I’ve seen for sale (I’ve only been intermittently looking for a few years, so maybe they show up occasionally, I don’t know). I’ve never bought anything on mercari and am not sure about signing up for another site to just buy one thing one time. Even though, again, it’s not a normal thing that you can just find places. Although I did find most of the rest of it randomly at a Goodwill for ridiculously cheap, so maybe I just need to wait for up to the rest of my life.

Anyway, anyone have experience with mercari? Is it OK?