I run a sporting events website where freelance photographers that have been to the events are free to upload their shots for sale, and I take a commission of anything they sell. Every photographer is free to set their own prices...
...except, when I created it I opted with PayPal as a payment provider and elected to impose a minimum price of £3 on each photo, as I deemed the fees for PayPal to be too high on lower prices (it was 20p + 3.49% at the time).
A handful of those photographers are asking me to increase the minimum price, as they think that it encourages rank amateurs to upload poor quality photos at £3 each, hurting their sales of (unquestionably generally MUCH BETTER) photos at (say) £7.50 each. And diluting the "good" photos on the site, making the site seem like a less-reputable resource for competitors to go to.
I'd like to increase it for the same reasons, and also because I expect it would increase my income marginally. Not by a lot, but to £4 or £5 maybe.
Currently, the mode and median price are both £6.50 and the mean price is £8.40.
Would this be classed as price fixing? Photographers ARE free to sell their photos to the competitors via other means (SmugMug, their own sites, or even give them away free on facebook etc)... but realistically most competitors DO come to my site in the first instance to find photos to buy.