If you are just looking to supplement your costs, buying new sets in store to flip is a pretty piss poor strategy. The competition is really high as anyone and everyone can get their hands on these sets. You need to hold them until after retirement in most cases to get decent value on them after paying for shipping and fees.
You're much better off buying bulk lots and splitting them up. You can get them locally on craigslist, facebook, offerUp, etc., and you can also find some pretty good lots on eBay and Mercari. Obviously you look for lots where instruction manuals are included, you can see lots of figures, and many partial/complete sets. Back before I had kids I was scoring a large lot weekly... and in many cases they were full sets all built in the pictures. If you nab lots like this on eBay, just ask the seller to individually bag the sets.
Take your time and do a lot of research. Find which older themes and sets are selling well. Be selective and patiently hunt for things.
I've made much higher profit margins buying bulk lots and splitting them, than I have clearing out Walmart and Target. I started with old Star Wars sets about a decade ago when they were still valuable, moved on to classic castle, classic space... more recently elves dragons. Collectors covet many of the older themes... no one is coveting the current lines as there is always a remake or something similar right around the corner... or it's so overhoarded that you can still get it for RRP on eBay 2 years later.
It is a good amount of work... but you get to build everything so that certainly adds to the enjoyment.