Bad luck, you should have stuck while the iron was hot, he probably told the buyer he'd got a bit of interest (you umming and ahhing, maybe 1 or 2 others) at £700 and the bloke did a quick bit of maths and figured it's worth a lot more so offered £900 to get it there and then.
It's good to see they're ('28's) holding some value as mine stands me at a (not so) small fortune.
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