In my experience there are 3 mains types of pricing.
"high street chains" - inflate their prices then come up with massive sales so pull the punter in. My view is avoid, I found their bathrooms and kitchens to be extremely poor value even during their sales. Advantage is they offer a full design, supply and fit service - convenience.
"discounted or specialist suppliers" - generally supply only but aiming a good value for money. e.g. IKEA or your local kitchen company, also covers internet merchants who are about shifting volume at lower margins e.g. Megabad. Some of these may have offers periodically (e.g. IKEA have 10% off all kitchens or interest free credit).
"genuine discounters" - shifting stuff that does not sell well or may be seconds. eBay is one such source. Some great bargains to be had (I am selling some surplus stuff at around half price, for instance) but supply is hit and miss, suits opportunistic purchasing provided you have the time and can store the materials.
I did most of my purchasing with the second group - googling for the best deals then getting on the phone and haggling where I could justify it - e.g. when I bought large quantities of oak floor, amtico, skirting etc. Most companies were happy to improve either the price or give free delivery.
I supplemented that with opportunistic eBay purchases, but given the time constraints these were minimal.
My biggest savings came from purchasing from Europe. I did this for bathrooms, rooflights, lighting as well as for some other bits and pieces. Google is your friend, but don't be afraid to ask for additional dscounts even when you find the best price, especially if you have some volume.