I think this would be a good idea. Nobody wants to be caught by a less than reputable online shop and we all want options and alternatives to the gw every now and then, so having a list of site addresses with brief descriptions from people who have used them would be a good point of reference for us all
I've used Darksphere, wayland, ttcombat, wargames terrain, wargamesEU, madrobot and Anvil, all have been great, no issues at all, and special mention for @Mad Robot , he went the extra mile for me when my package got messed about with by the royal mail and crossed the pond 3 times in total!
reseller wise, I find darksphere better value than wayland, but wayland tends to have a wider stock.
I've heard good things about goblin gaming too, but not used them personally.
For models, I've used:
Anvil- quick delivery and great for bits for conversion
Victoria Miniatures- mostly centred around the Astra Militarum but good quality resin models
Artel W- Delivery takes a few weeks but worth it for amazing quality unique models
For scenery I've also used Micro Art Studios. I bought an MDF apartment block from the Designed for Infinity range which was good and solid
As a UK based person: I tend to use Triple Helix as my go-to GW seller. Never had a problem with them. I've also used Goblin Gaming - there was an order mix up with them but the sorted it out right away and their customer service was top notch.
Beyond the GW product range I've used Hasslefree Miniatures a few times without any issues at all. Also gamemat.eu, Tiny Worlds and Vanguard Miniatures have been good.
I think Vertico is probably about the Tehnolog platform and Hexagon terrain.
I do not have any experience with the site, but they only take paypal for the payment and apparently ship with follow up (Except Australia and New Zealand).
And if done correctly, paypal grants buyer protection.
I get my GW (and other large manufacturer) stuff from Firestorm Games (my local), Goblin Gaming, Magic Madhouse or The Outpost for the most part.
As for non-GW, there's no small companies I have used that I can really speak ill of. They're all good for the most part. Some I have ordered from include:
gamemat.eu (game mats)
Deep Cut Studio (game mats)
Vanguard Miniatures (BFG alternatives)
Mighty Lancer Games (Reaper minis in UK)
Nannyogg (ebay seller)
Anvil Industry (minis and conversion bits)
Mad Robot (minis and conversion bits)
Dark Sphere (general games shop)
I use Element Games and Triple Helix as good sources of discounted GW products (Element Games also have a great paints, brushes, and tools department,) and I mainly use Bitzbox for bits (their customer service is excellent.)