He sells items needed for some quests. If you happen to lose one of those items, you can buy a new at his shop. However, since his prices are pretty high, you should be careful about losing a quest item in the first place. Note that Black Bert should be used as a last resort, not as a quick way through quests. Often, items bought from Bert will not work for the quests they are used in because the NPC checks for quest flags in addition to the item's presence. Also, some items are given by the quest NPC so you will obtain them through the quest regardless.
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