A couple of examples come to mind. Back in May 2021 there was a Roman Chair listed for Free. I emailed within hours and it was already spoken for. The next day it was listed for $40. Last week, there was a pair of Rep Fitness 35# bumper plates listed for $80 and within hours they were marked as sold and a few days later they were listed by the flipper for $140.
To some degree these folks are making a market. The seller gets what they wanted and a quick sale and the intermediate holds inventory to sell at a mark-up. There is some risk involved and some holding costs. Depending on location, the purchase could take an hour and dealing with lookie-loos and low ballers could take another hour prior to sale. That being said, a person could establish a nice side hustle by flipping $100 worth of equipment per week for $200 and bank $5000 a year for 100 hours of work.
A savvy market maker will not take a loss and there are near zero holding costs for weight plates and barbells. Bulkier items like a weight bench take up a little bit of room. The flipper Lucas Pederson has quite the selection of bars, plates and benches available.
When items are priced fairly. For example Iron Grip 45# plates and Kabuki strength 45# plates for $140 and $150 respectively, they tend to languish until finding a permanent home. $1.50 to $1.75 is high side fair. $90 would be a deal and $180 would be a rip off. Listed by Bryon Yang in Denver on 11/28/2021.
Christy Prehm has an interesting set up with squat stands, cheap barbell, 15# slam ball and 45, 35, 25, 15, 10 # bumper pairs and 2.5# change plates for $500. That is 260 pounds of weight from Simpsons Fitness Supply plus the stands and bar. If you set the price of bumpers at $1.50 per pound, you are looking at $390. Squat stands for $50 and barbell for $50 plus the slam ball and change plates is a pretty good deal. The cheap body solid barbells are $160 at Simpsons and even cheap squat stands will cost $200.