To add to what Chris said, minimums are preset for all sellable cards for whatever reason. Almost all singles and pros below PO tier are going for minimums now.
If you're just starting now, best bet to gain RP are daily goals and weekly quicksell (2500rp day/10000rp week max). And of course, grinding QGs, seasons and events (only handful left!) are main ways to improve deck. BTC can help a below Star/Playoff deck if you're patient... those BTC epics and legendaries are handy if you can get Wilds.
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