I think the signed-for vs second class is a red herring, because the seller was going to charge pinkandred £5.50, regardless of how she sent the parcel, and pinkandred is actually getting a better standard of postal service than the one in the listing. It would be very different if the seller quoted for second class post, but sent the item signed-for, and then attempted to charge extra for this.
As others have said, she has done this in order to protect herself from someone she considers (rightly or wrongly) to be a difficult buyer.
Elsewhere on here, someone is being taken to court next week, over ebay. She sold something, the buyer said it was broken, the seller said to return it for a full refund (including postage, as a good will gesture, though the listing says that postage won't be refunded in case of a refund), which happened, and now the seller wants more money - and has taken out court proceedings to get it.
Some buyers and some sellers are totally barking, and maybe this seller has been burned before, by someone less nice and decent than pinkandred, and that's why this has raised red flags for her, and why she has sent the items signed-for.