By Laura McGuire Personalised card company Moonpig suffered its worst day of trading in three years on Thursday, after a private equity firm sold 25 million of shares in the firm. The move sent shares in the FTSE 250 plunging resulting in what was the worst day of trading for the company as a listed business. Investors led by private equity firm Exponent sold 25 million shares in the card and gift business at 160p a piece, more than 10 per cent below Wednesday’s closing price of 178p. The sale cut Exponent’s stake in the business by about a third and raised around £40m in process. Strategic Va…