I think the forum classifieds have died down a bit, (there's a whole thread about it), this is probably down to a number of factors, one is that compared to 5 years ago (or more) there are far more places that will buy your
second hand kit from you, or offer you a trade in (places like
MPB or CEX) and there are way more classifieds/trading sites, especially now via Facebook where there are numerous selling groups (I'm in 3 photography equipment selling groups myself). Of course ebay is there and even Amazon can be used to sell gear (some saying it's better than ebay). So this leads to fewer adverts on the TP forum.
I did buy my 6D from Calumet, but that was only after spending a few weeks watching ebay, searching the TP classifieds daily, searching Facebook groups and every retailer going. In the end I found mine by walking into the store and spotting it on their shelf.
I've been told by both the guys in Calumet and
London Camera Exchange that it's often the case of trade-ins come in and go out so quickly that the effort of photographing and cataloging the items isn't worth it. Hence why those shops don't have as much listed s/h as the actually have in stock.
Due to upgrading, I was trying to sell my old 60D and 2 lenses, after days of advertising on here, I got no interest (despite the ads having been viewed many times) and ended up selling 1 lens via Facebook, 1 lens via eBay and then the body to someone I knew.
(Also as a slight aside, the new app doesn't give you any alerts, so you don't get notified if someone comments on your for sale advert).
I definitely think the 6D is worth the extra £150. A £550 5D2 is likely to be older, heavily used, higher shutter count before you even think about the spec upgrade. Having used a 5D2 a few times in low light, the 6D definitely has better performance.
But as you say you're mainly going to be using it in a studio situation, the low light performance may be less of an issue.
When you are on a limited budget, you have to go with what you can afford out of what's available at the time.