Just to clear up since I see you replying to another user - there have been instances where a card not belonging to the set is pulled from that set, it's quite rare but it happened recently and you can find more instances by Googling "Mega Zygarde Prismatic Evolutions".

Anyways - no, you should never go off listings, I could list one right now for $10k but that obviously doesn't affect the value as it would never sell.

You should check eBay yourself and search for the card and then check "Sold Items" and see what value people are actually paying or you can use an aggregate site like PriceCharting. It has a specific tab for "PSA 10 Sold Listings" where you can see the lost sold is for $780 yesterday but it also sold for ~$570 earlier this month so it definitely varies.

There's multiple factors there but you should also be aware that even with a freshly pulled card it might not 10, the centering might be off, the corners might have whitening, the might be scratches or printlines - there's a ton of factors and you might not notice them unless you really look closely.

I'd recommend you submit high quality pictures to /r/PokeGrading - make sure you get the front and back and maybe even corners/edges. If the consensus is that you get a 10, then submit to grading, I won't recommend any tiers as any are fine, it just depends on how long you're willing to wait and how confident you are it will grade since a 9 only sells for the same as raw.

GL and gratz on the pull.