Rebrickable does not sell any LEGO parts directly. We link to external stores that sell the LEGO parts. We have access to these stores' inventory and so will only list stores that have relevant stock at the best prices.
There are many places on Rebrickable where you can buy parts - anywhere you see the 'Buy Parts' tab. These exist on Set/MOC inventories, Build results, Custom Lists, etc.
There are several switches you can use to filter the search for stores.
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