I bought my first sight-unseen bass at about 17, back in the 90s. Got a custom Carvin 6 string because i was pretty sure i needed a 6 string (Thanks Les Claypool and Anthony Jackson and Bass Player Magazine.) And i was amazed at what you could get for around $1K. Saved up my money from my crappy minimum wage job and bought one. I still have it. If i had played it first when i was 17 i might have thought twice about it, but i probably would have written off the weird things i don't like as "things i have to learn on a 6 string." If it was today and i played it first, i probably would have gotten a different bass because i'm (hopefully) smarter now, and have a lot more experience behind me, and have the experience of buying the Carvin behind me, and now knowing all the things i thought i wanted vs all the things i really want.
Fast forward to today. I have a deposit on a new Maruszczyk Frog fretless, sight unseen. Feel more confident in the decision because, again, i have decades more experience and bass wisdom behind me, and i know exactly what i want from a bass, not what i think i want.
I ordered a custom Olinto P bass too. So technically i haven't played the one they're making for me because it's not built yet. But i ordered it because i fell head-over-heels in love with the Olinto P they had on the wall of their shop. So, i have played one first, just not mine specifically.
Used? No. I wouldn't buy a used instrument sight-unseen unless it was cheap enough that i wouldn't care if i had to put some money into fixing it up to how i want it. But, chances are, if i'm buying used, sight-unseen, it's gonna be something i'm specifically buying to not worry about modding. I don't even care if Reverb.com et al have buyer protections and i can send it back. I don't even want the hassle of finding out, filing a claim, and shipping it back.
Otherwise, it would have to be sight-unseen from someone i absolutely know and trust. I can think of a very small handful of people i would, and one of them lives close enough to see the bass in person.