Like anything else you buy from one venue and sell in another. You buy wholesale and in bulk, and sell one at a time or break it down into components. I don't try to get rich on anyone. Some deals are better than others, sometimes break even is a good thing.
I always attempt to fly or run whatever I bring in to have personal knowledge of the item being sold, goes a long way to CYA.

I've bought from Towers scratch and dent (box damage only) ads for years, that's a inventory of $34,000 a year I turn over typically.
Who flies the box? If it's missing hardware, great, $4 in that and I'm ready to retail it and they knock off some $50-100 most of the time, and I'm using the same club coupons you use and free shipping. I picked up a Stearman for $216 to my door and listed it here for $275, it sold the same day it arrived. All that is missing is the windscreens and he was fine with a $4.59 item he can get when it's back in stock.
Swap meets are well known for bargains. Look up Mirza in member search, he beats me to a lot of deals cutting in front of me! LOL

I'm in Florida, it's a retirement area, lots of estate sales, many eventually have health get in the way of their hobby and their collections are an accumulation of 40 years in some cases of beautiful planes that just need a general going over and new batteries to fly.

I started in 2002, I mentioned earlier on page 1 or 2 how I got started with $600 and bought 17 used planes. I started on E bay and discovered RCU's marketplace a little later on. I liked E bay for the auctions, the buyer determines the price, not me, but RCU has a blue book value history that is an excellent tool for me to buy and sell with as a guide so everyone is happy with the deal.

I do not buy new equipment for myself. My 10C radio came from scratch and dent, it was $375, I sold the 14 channel receiver for $110, and my 9C radio for $250. I only bought one dead radio in all of these years, the rest worked as it should.

Sebastian holds a swap meet in a parking lot in front of Riverside Hobbies twice a year and I am the first and last one there.
That's where I bought a Telemaster 40 rtf with Saito 56 for only $180. I love using flaps and watching air speed move it backwards on windy days. The Astro Hog was also bought there. I still laugh at one year I bought everything I saw all morning long, spent $2000. Then waited for everyone to leave. An hour passed and the afternoon Church crowd in dress clothes rushed in to buy and I laid out everything I just bought and sold it all for $3000!

One guy gets out of the hobby and he doesn't want to piece, part and ship everything. I do! I use greyhound quite a bit with great success. I sell locally at the flying field, when they crash, they come see me. I buy from guys giving up the hobby and they usually set the price for their garage full of planes. I never spent more than $1400 for a large collection of planes that in a few days I recover that expense with just 4 or 5 planes sold on RCU, and another 10 - 15 planes remain for an income, personal use, and gift giving.

I look on Craigslist daily. People just want to get rid of planes for a fraction of their value. I had many ARFs I bought that wouldn't sell for years until they became discontinued. Sometimes you wait for that and you can get your price.
I find kits to be the best swap meet deal you can find and on RCU or E bay, it brings in twice or three times the money.
Airborne-models.com has overstock sales continually at half price, and Hobby People have sales from time to time especially around xmas.
Enjoy.