Merchant inventories are reset each time you go up a level, and are generated the next time you talk to that merchant or run your crosshairs over them. Since the equipment changed I assume that was the first time you talked to Derk since you last leveled. You will not be able to get those exact pieces of equipment back, but if you reload and trade with him a few times he may offer you something similar.
Don't waste too much time reloading, though. Unless you really need something upgraded, you're probably better off just playing, and checking loot for upgrades.
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