I had the same text and the same thoughts about this offer. if you go on www.offers-autotrader.com and look at their Terms and Privacy page you will see the following :

"9.a Our Feeswww.offers-autotrader.com does NOT charge any fees for our service.We may request a fully refundable appointment deposit. The deposit will be returned to you at the appointment - even if you do not decide to sell your car to us."

Also if you check Autotrader's fraudulent buying section you will see their advice:

"Fraudulent Car Buying Companies.Sellers will receive a text message from a company offering to buy your vehicle for a reasonable price. Sellers are then invited to go online and enter their vehicle details.You will then receive an email with your valuation asking you follow a link to a website. You will be asked to fill in your details so the company can collect the vehicle. You will be advised that a “refundable” deposit will have to be paid via a Pay Point to complete the sale.You are told that you will receive the refund on collection of your vehicle. The reality is no one will collect your car and you will not get your money back."

Agreed, why would you pay them? Absolutely a scam!