As a kid I once visited a friends home and took some of the marbles that belonged to him.
How do I do teshuvah for this since I no longer have the marbles or know their value.
Answer:
You can buy him a new bag of marbles (for him or for his children).
This will certainly put paid to any lingering fears.
Note that if you stole the marbles as a minor, there is a dispute as to the obligation of returning the theft, but if you remember the incident clearly it is right to do so.
Best wishes.
