I second this. A great strategy to get started is to go with used retired sets. Listings appear daily where someone is just quickly unloading their collection, or someone's kid is done with Lego and the parent just dumps it. I haven't bought too many used lots this year since the baby was born (no time to sort and build), but I would pick something like this up 3-5 times a week in the years before. It was by far my highest profit and lowest risk. Plus you got to enjoy the building experience.
The few examples from this year:
Contained a complete (remainder of parts in the basket) 75054 (AT-AT), 75050 (B-Wing), 75092 (Naboo SF), 75105 (MF), and 75094 (IST)
^^ Contained: (3x) 7595-1 Army Men on Patrol, 7667-1 Imperial Dropship, 7668-1 Rebel Scout Speeder, 7676-1 Republic Attack Gunship, 7751-1 Ahsoka's Starfighter and Droids, 7914-1 Mandalorian Battle Pack, (2x) 8014-1 Clone Walker Battle Pack, 8018-1 Armored Assault Tank (AAT), 8083-1 Rebel Trooper Battle Pack, 8084-1 Snowtrooper Battle Pack, 9488-1 Elite Clone Trooper & Commando Droid Battle Pack, (10x) 30071 Army Jeep
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Another highlight for $125 shipped: 6212 X-Wing Starfighter, 8128 Cad Bane Speeder, 75084 Wookie Gunship, 7930 Bounty Hunter Gunship, 75145 Eclipse Fighter, 75024 HH-87, 75091 Flash Speeder, 75167 Bounty Hunter Speeder Bike Battle Pack, 7914 Mandalorian Battle Pack, 75131 Resistance Trooper Battle Pack, 75133 Rebel Alliance Battle Pack, 75164 Rebel Trooper battle Pack, 7957 Sith Nightspeeder, 75022 Mandalorian Speeder, 7965 Millennium Falcon, 8016 Hyena Droid Bomber, 9498 Saesee Tiin Starfighter, 8085 Freeco Speeder, 75180 Rathtar Escape, 75148 Encounter on Jakku
If I had more time I would go back to doing most of my business with buying bulk retired used sets, separating, and flipping them. You can make a good surplus this way to save up to start clearing shelves off without going into debt.