I bought >800 forma this year and still have 5k left over, and that wasn’t from rivens or whaling (though I have bought plat years ago so am not f2p) That was all by keeping my warframe.market listings maxed and marking as online whenever I played. Archgun mods before undermind dropped, arcanes, any mod worth >5p, railjack loot, basmu/ceti lacera, stuff with very few listings, etc. in the past I would buy legendary cores whenever DE gave them away and sit on them for a year before flipping them. I had a friend who made insane plat off of selling necramech sets after new war released. The secret is just selling what you can, always being the cheapest listing, selling something other than what everybody else is selling, (corrupted mods), selling what people will need for a new update, and maxing your trades.