I've been playing d4 since season 1 and always seasonal. Once the season ends everything moves to eternal. I have like 8 seasons worth of stuff.. im not a hard grinder. For the most part, once I got the season done I stopped and played something else.

So tl;dr what's the best way of selling it for gold then go fg or can I sell the item for fg?

Selling something on eternal d4 looks like unreal task to me, so clearly best way is to sell everything before season ends.

This. The only thing of value when it comes to fg on eternal is gold. Everything else is a waste.

After every season, I nuke each character unless I have something like a 4GA Mythic {never gotten one}, I salvage it all to see the amount of obducites and shards and whatnot climb. On Eternal, I'm up to 300k obducites.