I tried to order a set of textures from the unity asset store for the first time the other day, only to have it reject two different credit cards. I called up VISA (one was VISA one Mastercard) and they said they saw the transaction, and it was accepted by them but canceled by the merchant. I tried sending Unity a 'sales inquiry' through their website but they haven't gotten back to me. Anyone else have a similar problem? Any ideas on how I would go about resolving it?

Never had any issues buying from the store but you definately should contact the asset store (and not sales inquiries which is probably only for sales relating to unity itself and not third party items on the store). Also you'd be much more likely to get an answer on the unity forums that on gamedev (including likely an answer from the staff there).