Battery first time charging duration
I would ignore the seller's advise and follow the manufacturer's advise to charge for 24 hours when charging for the first time. The seller didn't design the product so how can they know better than the manufacturer?

What can happen is that after a long time in storage the NiMh cells lose their charge completely. NimH cells can handle that. After the full discharge NiMh cells can be "revived" by just charging them slowly. Many products always charge NiMh cells slowly because that is the cheapest, safest and easiest solution.

When the cells are fully depleted it might take some time for them to "recover" especially when charged slowly. That might be the reason to charge for 24 hours. That way the cells have a couple of hours to regain their nominal voltage and when they do there is still enough charging time to fully charge them.

Lithium based cells generally should not be fully discharged so they need a protection circuit to prevent that. Also Lithium based cells do need a proper charge controller, just trickle charging them continuously is not recommended as these cells cannot handle that very well.

So charge control and full-state detection as mentioned in a comment only applies to Lithium based cells, not NiMh cells. For NiMh cells charging can be done safely using a resistor and a diode at C/10. Charging then takes long but completely safe.