FWIW with Activesport it wasn't just that they didn't have the item in stock despite telling me they did... It was that they then repeatedly lied about it, and not even good lies.
Did it to me twice (didn't realise Mark Anthony Bikes and Activesport are the same company...). First time was a set of salsa bars, which they claimed were in stock, then would take 2-3 days, then had been posted, then must have been lost... Then when I finally complained to Ebay, they cancelled the sale, and announced that it was my fault as I hadn't specified the colour. The bars only came in black.
Second time round was a stem... Again, promised it was in stock, then oops, sorry, will take a few days... After 2 weeks of that, some snow arrived, so then "Well it's not our fault, we'd totally have had it with you by now except for the snow!". I mean, don't get me wrong, I've lied to customers but they've always been good lies! Don't treat me like an idiot, don't act offended when your obvious lie turns out to be obvious... But most importantly takes some pride in your work damnit! Be the best liar you can be!
The final touch was when I left them negative feedback, and they emailed me with a bribe to try and get me to withdraw it.