I'm inclined to believe that at that point, Tina was still alive. Whatever they bought must have something to do with one of two things: the homicide (which is why they won't discuss it if it if they anticipate a trial) or something to imply a sexual relationship (condoms, etc <---- just speculation on my part). Otherwise, if they had bought something innocuous like food, then LE would have no problem saying that. Or clothing or hair dye, they would say 'may be in such-and-such clothing' or 'may have dyed hair'. KWIM?
There's something about the purchase that they want to keep a lid on.
Also, if Tina were already dead, I think they would have been on the run and not at Wal-Mart.