Starfield Survey Data: Where to sell Survey Data, and why
One of the less obvious mechanics in Starfield is gathering Survey Data - if you've ever looked through the Notes section of your inventory and noticed Survey Data, no, you didn't just pick that up and forget about it. If you complete a survey of a planet, by scanning all Flora, Fauna and Features then you'll have the Data added to your inventory. These are completely useless, and are intended to be sold. While you can sell the data to the Trade Authority, the most efficient way to take advantage of them depends on if the data is for an Inhabitable planet or an uninhabitable one.
Where to sell Inhabitable Planet Survey Data
In Cydonia, on Mars, players can find Phil Hill - a worker at LIST, and who enlists your help in finding data about habitable planets so he can help match applicants with the best planet to fit their needs for starting their new lives; once you've completed Survey Data for a planet that can support life, simply head back to Cydonia and talk to Phil to sell him the data slate.
Since LIST is all about helping settlers find planets to live on, he won't pay you for survey data for uninhabitable planets. While you can still sell those slates to other vendors, thankfully there's another option for players to consider.
Where to sell non-Habitable Planet Survey Data
While you won't be able to sell uninhabitable planet data to your buddy Phil, Constellation's own Vladimir will take any and all survey data off your hands - for a better price than regular vendors, too. You can find him either in The Eye, or in The Lodge once you've progressed the story to a certain point. Be careful, though; since Vlad will buy and and all survey data, make sure to sell any of your inhabitable planet's data to Phil first before speaking to Vald, just to be safe.
One neat trick that you can pull with Vlad, is with gas giants; since he doesn't discriminate with what data he'll buy. If you enter the orbit of a gas giant and scan it - which will automatically get you its survey data - you can go around scanning as many of them as you can, before stopping by Vlad to sell them. Since he'll give you a little more than 1000 credits per data slate, if you're ever running low on cash it might not be a terrible idea to go around scanning a few gas giants before returning to him for your reward.
Unsurprisingly, players will likely end up with survey data without actively looking for it - for example, when they're going through the process of tracking down Scanner Anomalies - and it goes without saying that if you can spare the time, it's worth selling them to the right vendors. Best of luck, Constellation!