$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.