On the other hand, let’s now talk about the Frontier mode and for those who are hearing about this mode for the first time, it is the newest mode in splinterlands which is completely free to play

If you are playing in free mode you can't sell anything. Even when you buy the game you can only sell frontier cards without "investing" (read funding the wealthy) and they have been made worthless due to the wealthy (including the multiple bots of the wealthy) buying their way to the top of the league early on and getting all the bonuses for doing so.

Until opponents are matched by power (owned, rented and delegated) it's not worth buying the game.