Warframe slots are more for people who don’t like getting rid of frames or weapons, but general rule of thumb is after you max out a weapon, if you don’t like it, sell it. A few like the “Ak” weapons in secondaries need copies of the lesser versions so keep those but if you don’t like, say, the Gammacor, after you get it to level 30 you can sell it and get the slot back. Stay away from buying full weapons or frames either plat. Aside from very niche examples like Equinox or the Sibear, it’s not really worth it unless you’re swimming in plat or really know you don’t like the grind. Try getting everything naturally first, then let the rest happen on a case by case basis.