The new owners were leasing from month to month. When they moved out you had the right to take possession immediately. If there was any doubt, the matter was clarified when they handed you the key.
It appears that they abandoned everything left in the store after delivering the key. This stuff appears to have little value (except possibly the computer). Move the property off premises and give them 30 days to claim it, provided they pay a reasonable storage charge. Tell them that you will dispose of the property after that time. Send the notice certified and keep a copy.