Question on using already seeded filter

I bought an aquarium last week that came with a fluval 405. Is it recommended to replace all the media and start seeding it myself or use the media that came in it? I have no idea what he kept in the aquarium because the fish were already gone. It's also been sitting dry for a week

If you don't know anything about the previouse fish that were in the tank I would start from scatch. You sure don't want any bad stuff in a new tank. Cleaning the tank out completely and use a new filter medium. Cycle it yourself. If you have a filter from a tank you already have set up and running you could use that to seed it.

That's what I was thinking. Definitely don't want to risk anything

especially since it sat for a week without running. it probably went anaerobic in the canister

Yup unless you are seed from your own tanks and you know what your fish have/don't have I wouldn't use another filter.