Is it permissible to sell virtual currency earned from electronic games for real money? And is it permissible to involve a third party in a bet within the game, whereby they enter the contest on my behalf in exchange for a percentage of the profit in case of victory, while I bear the full loss in case of defeat?

If the game is permissible, then there is no objection to selling or buying add-ons within it (such as virtual currency). However, betting on these add-ons is impermissible and is considered prohibited gambling. Also, involving a third party to enter a fight on your behalf, and you give him a percentage of the money earned if he wins, while you lose money if he loses without him losing anything, this is also prohibited gambling. In this case, the third party is an agent paid from the betting عوض (recompense/stake).