That could be possible in smaller locally ran ones. But the bigger national ones most likely (Goodwill, Saint Vincent, Dessert, Salvation Army) have professional pricers. The store I worked at didn't have any volunteers. The prices are pretty much set.
Bike for example be something like this.
Kids Bikes: $10, Adult Bike $20.00, Better Bike $40.00.
From there the prices would be reduced or increased based on condition accessories, and information about the product that the pricer knows. You don't get research time. you got 5 seconds. At best, a minute to consult with other pricers. Also if another pricer notices your pricing is too low, they can dispute it and then the manager would get the final say.
So a new looking Huffy with a basket, bell, and a rack would cost say $30.00 ($20 base +$10 (for accessories)) could get a higher price than say a 60's Raleigh Sports with rusty rims, a flat, and a beat up saddle say $10.00 ($20.00 base -$10 (for condition)). Of course that would depend on the fact that the pricer wouldn't know that Raleigh is a better brand than Huffy, and that they are unaware that their exists a collectors market for the Raleigh vs. Huffy. It's not like they make these prices up out of thin air, there is a system. So a guy couldn't just go in and say that Fuji track bike that came in is only $5.00 bucks then buy it. If it looked too nice to fit into the pricing system, it went to the card file people, from there the manager might research it, or all the pricers discuss it, or perhaps even call in multiple card file guys to auction between them in a day or two.
When I did this, it was before the internet took off. (Early 90's) and to some extent on line auctions have replaced the pricers in some product categories. Set a low minimum bid and let the market price it. But big items take up a lot of precious floor room and back room space. Here they are less picky about price and more concerned about moving it out. But in the early 90's - with no ebay or craigslist I couldn't have made much money reselling. I was at work during retail hours, and no easy or cheap way to advertise. But I'm sure that they've developed policies to try to prevent such things.
They also have policies to keep employees from getting too much of an advantage over the public. At our store we weren't allowed to purchase on company time (could only purchase during our lunch). Plus we had no discounts, items not on the floor had to be confirmed with the manager, thus pricing fraud is curbed. Our only real advantage we had over anyone else was first pick, and just being there every day.
Plus we had great pay (more than double typical retail at the time) and benefits. I don't think any of us were willing to loose our job over a couple extra bucks here and there. Why loose your job over an extra $100.00 reselling some old bike, when you never know if a first edition of "Leaves of Grass" might stumble through, then I'd perhaps chance my job.