Good luck with your project!
I can only speak for myself of course but regarding buying trends I tend to knit more in the winter than the summer. I tend to sew more in the summer.
Although I am sure some avid knitters knit all year round!
I do sometimes knit in the garden in the summer but if it is very hot I don't knit so much - and I don't like to have the extra layer of 'clothing' in my lap in the heat especially if I am sitting in the sun. I prefer to be holding an ice cream!
For other people they might be knitting for new school uniforms needed in September so school uniform colours of black, navy and red, etc. might be quite popular coming up to that time.
I think if you were starting a website for selling yarn it would be very important to link yarns with patterns so that potential customers could get a good idea of what yarns would look like made up.
People often buy patterns and yarn at the same time as well.
I think also it is very important to have the best photographs possible on your site because yarn and fabrics are the kind of product you would normally like to touch before you buy. The colour representations should be very true-to-life as well otherwise you could get disappointed customers.
Also I think if you were a small business selling alpaca wool it would be really lovely to have photographs of your animals and their names! I think the customers would love that.
I think if you ever want to start any business you will get more helpful ideas from asking potential customers than any business advisor so I hope you will get some good feedback from this forum to help you.
If I think of anything else I will come back to you.
PS. By the way I never buy real wool because it makes my skin itch.