$600 looks to be the most reasonable price for a sealed new unlocked Galaxy S9 [in the US]. The lower priced 'new' copies on Swappa look open-box, which are likely not new, but a refurb/return, or someone selling their used device as new. Most of the new copies on Swappa are apparently open-box units, as are most of the new units on Ebay. Definitely hard pass there.
Best Buy has new copies for $700 - that'd probably be the... best buy. Maybe you can find a like-new refurb for $500, but it seems like a sealed actual new copy will be harder to find for that price.
So like $150-250 saved by choosing Samsung, and probably getting a superior package in most ways. Google has reduced the warranty period in the US of the newest waves of Pixels by 1 year, so really they aren't offering a ton of value for that high price.
The NA model is still $720.
At the same time - yes, B&H gives it their own warranty.
I am only seeing them ~$800 (not plus or note which are obviously even higher), e.g. from AT&T [0] and Verizon [1]. That's quite the difference from $500, can you link where you are seeing this or do you mean second hand or when you switch carriers or something?
0 - https://www.att.com/cellphones/samsung/galaxy-s9.html#sku=sk...
1 - https://www.verizonwireless.com/smartphones/samsung-galaxy-s...