I think the answer is "it depends" and there's no right or wrong answer, really. Take a look at the sales figures on the sets you have (by finding the set's catalog entry and looking at the pricing tab) - are people even buying the set? If so, are they paying a price you are willing to sell for? How long are you willing to sit on a set to get the price you want?
Lower-demand sets may be worth parting out to recoup your money faster. Higher-demand sets may be worth selling as-is (sealed) if there is turnaround and at a price that makes you money. Is the set still in production? You won't get as much $$ for it clearly if anyone can go buy it retail right now. Conversely, some sets are worth sitting on several years and then listing later, such as Creator Expert modular buildings (if you're wanting to go after higher $$).
Another good source for finding set demand is brickpicker.com; that gives you a view of set turnaround and prices paid across multiple platforms (e.g., eBay, BO, BL, Amazon), by month or by year.
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Lower-demand sets may be worth parting out to recoup your money faster. Higher-demand sets may be worth selling as-is (sealed) if there is turnaround and at a price that makes you money. Is the set still in production? You won't get as much $$ for it clearly if anyone can go buy it retail right now. Conversely, some sets are worth sitting on several years and then listing later, such as Creator Expert modular buildings (if you're wanting to go after higher $$).
Another good source for finding set demand is brickpicker.com; that gives you a view of set turnaround and prices paid across multiple platforms (e.g., eBay, BO, BL, Amazon), by month or by year.