I have read that only actions that are free from the 8 worldly concerns generate merit.
Actually, even actions contaminated by the eight worldly concerns may generate merit. For instance, the action of saving a life may be contaminated with the side thought of receiving praise, but still is a virtuous action, and does generate merit.
In general, the enjoyments of the desire realm, which are merits nevertheless, are caused precisely by virtuous actions, contaminated however with desire realm afflictive mentalities, and therefore with attachment for this life, and thus with the eight worldly concerns.
Technically speaking in my opinion no un-enlightened beings in samsara are capable of actions that are free from the 8 worldly comcerns.
Actually, even samsaric beings abiding in higher (form and formless) meditative states are free from the eight worldly concerns, even if only temporarily.
Because we are govern by the 8 worldly concerns we remain in samsara.
Giving up the eight worldly concerns is necessary to achieve rebirth in the upper realms, but not enough to quit samsara!
In order to quit samsara, there is the need to give up not only the attachment to, or concern for this life (which includes the eight worldly concerns), but also attachment to samsara in general.
Besides, the correct view of reality is also necessary for liberation from samsara.
So merit is actually extremely difficult to collect. Good karma on the other hand is what we mostly collect by our good deeds and virtues acts.
Merit by definition, is the result of good deeds and virtuous actions. Therefore, collecting merit is as easy or as difficult as doing good and acting virtuously.
I think the Buddhas do generate merit but not like us that they gain merit per say and definitely not for themselves.
Sure! The merit buddhas create is pure, because it is thoroughly free from afflictive mentalities together with their seeds, and even their predispositions, but also it is vast, because is fully dedicated to all sentient beings without an exception.
The virtuos acts of Buddhas are free from the 8 worldly concerns and it is the Buddhas that we dedicate our "virtuos actions that are not free from 8 worldly concerns" in order to gain merit instead of just collecting good karma. Just my two cents.
Dedicating our virtuous actions or karma to the achievement of buddhahood for the benefit of all sentient beings, the result is a merit so vast that it will not be exhausted until the samsara is empty, and all sentient beings without an exception are established on the ground of buddhahood.
And, even if our virtuous actions are still tainted by the eight worldly concerns, these and other evils will be burnt by bodhichitta, as explained by Shantideva in his
Bodhicharyavatara.