Be very wary when booking so called "specials"

My partner and I tried to book a trip to London. As we were paying on seperate cards we sat together each on our own laptop to make the booking. Everything went well until we wanted to make payment. My card was accepted but although my partner had more than enough for the fare his card wasn't accepted. We then tried another card and also wasn't accepted. He started the process again but the system indicated no availability on the chosen flight. He then had to book with the airline at a higher cost so we were on the same flight. However a couple of hours later Travelstart was showing there was availability of the chosen flight....... What is going on with Travelstart? They offer "specials" but than travellers can not book them.......