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Step 1: If you obtain an item at a very low price, either keep said item, or sell said item for an even lower price.
Step 2: This is not a baseball card collection. Signed items, (refer to example B), are not anymore valuable than an item that is unmarked. In fact, the autograph probably makes said item less valuable.
Step 3: Tramdock, Brociety, Whiskey Militia. If you buy an item off of one of these sites, refer to step 1. No one likes a flipper.