Analysts say the firm, which also on Wednesday posted half-year profits of £42m, would be worth up to £5bn.
A £200m debt pile and difficult trading conditions have forced Countrywide to raise funds to stay afloat.
The “consensus view” at the US Federal Reserve is that gradual interest rate rises remain the best policy, the head of the US central bank has said.
The Competition and Markets Authority will look into whether the deal will mean less consumer choice, higher prices or worse service.
By the end of trade, the benchmark index is set to have gone 3,453 days without a fall of 20% or more, marking the longest such rally in US history.
The London-based firm, founded in 2008 by Portuguese entrepreneur Jose Neves, sells designer clothing online from firms including Gucci and Burberry.
After the biggest bailout in global financial history, totalling more than €260bn, the country will be paying loans off for several decades.
B&Q owner Kingfisher saw a 1.6% rise in like-for-like sales during the second quarter, helped by recent warm weather.
The Office for National Statistics (ONS) figures also showed that retail sales rose by 3.5% in the year to July.
Inflation remained at 2.4% for the third month in a row in June, according to the Office for National Statistics, after clothing prices fell.