If you're unsure about upgrading so soon after buying the T1, why not just wait for at least another couple of months for more feedback on the Glo. That way you'll have more idea about it's good and bad points, they might be onto a second batch of them (so possibly less chance of getting one with any issues?) and your Sony will be that little bit older so you don't feel so bad about getting rid of it!
Also, there's more chance of your local WH Smith having a demo model in if you wait for a while. I've visited two stores in the first week of it's release, both have contacted head office about getting a demo model but have no idea when (or even if) they'll get one. The bigger shop had the older Touch, WiFi and Vox demo models out on display, but Glos and Minis on sale.
As for selling it, well there is the flea-market here but for the widest audience you can't really beat ebay. You could always try selling it on gumtree first, and if you get no takers, then fall back on ebay.