Pistoliers

Action : Spend 1 resource to move this unit from it's current zone to another of your zones.

Nurgle Sorceror

Quest Action : Spend 3 resources to deal 1 damage to one target unit.

Can I perform these actions as many times as I have resources to pay for them?

Yes. As long as a card does not include a limit, nor requires a specific triggering event, you may use an ability as often as you can meet its cost.

When an effect requires you to meet a specific triggering event in order to initiate it you are restricted to one trigger per event. Read carefully what the triggering event is, sometimes it can seem to be something different than you may first assume.

Two more examples of multiple action use are the Dwarf cards

Runesmith - Action: Spend 2 resources to have a target unit gain pow* until the end of the turn.

Grudge Thrower - Action: Spend 1 resource and sacrifice a unit to have each attacking or defending unit gain pow until the end of the turn.

Compare those to the High Elf card

Archmage of Saphery - Action: During quest phase, you may heal 1 damage on one target unit. (Limit once per turn.)

The Archmage card specifically tells you the action limit, the others are unlimited. In the case of the rune priest, if you had 6 resources you could give a unit +3 pow.

* in our local games we've all gotten into the habit of referring to hammer icons as "pow".