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Not sure on the T charge, if you're not planning to keep it until the Ulez comes in then it's probably going to be easy enough to find something that complies. Looks like you could definitely get a modern SH125 for £1700, probably less in a private sale. Not sure if the winter price effect really kicks in for a month or 2 though.
Just be careful with private sales in London, I've turned up to a few that were so dodgy I walked away. If you're going somewhere with enough cash to buy the bike you need to watch your back. Never had a problem with big bike sales but scooters seem to attract the scammers.
Having said about the T charge not being a big problem, it's obviously going to affect resale value as anyone looking for a scooter for a few years won't be interested unless it complies with ULEZ.
Maybe you want something fairly new if you want to avoid the incoming ULEZ rules? I don't have any little ones to sell but the SH125 is a decent scooter. You should find some a few years old for reasonable money. Motorcycle Giant have a couple for sale. Not sure what your budget is though.