Yes the info posted about stamp duty is incorrect.
But, it is worth mentioning that anything over £250,000 will incur a 5% charge for stamp duty (for the portion over £250,000). So stamp duty on a sale price of £250,000 is £2500, the stamp duty at £280,000 is £4,000.
So its not just the difference in price (of say £30,000), it's the extra £1,500 that you'd have to pay for stamp duty. That's for ordinary residential buyers.
If you're looking at BTL investors (which you mention quite a lot) don't forget that any sales that complete after 1 April 2016 on 2nd properties (or 3rd / 4th etc) incur an extra 3% stamp duty. So, after 1 April, at £250,000 they'll be paying an extra £7,500 in stamp duty (it goes up to £10,000) and at £280,000 they'll pay an extra £8,400 (it goes up to £12,400). You still have 2 months to complete before 1 April, but I would anticipate that lots of buy to let investors are conscious of the changes and are factoring that into their offers.