I think the level 20 items you see in the beta are just to give you an idea. Bungie probably doesn't want spoil everything yet in the beta. This is a special build, so there are probably even a lot more level 1-8 weapons and gear we didnt even see yet.
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i think the lvl 20 items we see are the lowest legendary item the vendor sells, the store changes with youre level ( being level 8 shows you items of lvl 6,7,8,9,10) and since youre below level 20 they show you the lowest legendary wich is level 20, i think that this is how the shops work, atleast in the alpha it was like this didnt really pay attention to it in the beta