In selecting an Athame (AH-thah-may, or in some Traditions, ah-THAH-may), there are some important considerations to take into account before selecting your tool.
Tradition: Many Traditions have entitlement requirements for certain tools. If you are training in a Tradition that requires that you take initiation before owning an Athame, then it's advised that you wait until that time. If you come across your PERFECT tool that you "know is yours" before you are entitled to own it, talk with your teacher, who may be willing to hold onto it for you until you can rightfully wield it yourself. Also, many Traditions have guidelines for tools, such as requiring that the Athame be made of steel or that the handle be a certain color or material. Some even have blade length restrictions. Be sure that you know the restrictions of your Tradition before purchasing your tools.
Legality: You may want to own a 7-inch stiletto as your Athame, but if the laws of your community forbid owning it, then it's not a good plan to have one. Never assume that no one will find it. You could be on your way to a community Sabbat 9 years from now when the police start searching all vehicles because they're looking for something in particular. You can be fined for having such an illegal object.
Practicality: Selenite is a beautiful stone and I've seen selenite Athames for sale. It disintegrates in liquid and is poisonous. So doing a Great Rite with a chalice of wine and a Selenite Athame is a bad plan. Be sure that if your blade isn't steel, that it doesn't poison wine or dissolve in it.
Ethics: Every Wiccan or Pagan who uses an Athame and other tools has a different understanding of ethics in their Craft. If you're a vegan, don't buy that cool blade unless you determine if that awesome decorative inlay on the handle is real Mother of Pearl, or you're going against your own ethics. That's just an example. Many Wiccans want to be certain that their tools are ethically sourced. That means that the lovely lapis jewel in the handle end can't be mined by slave labor, for instance. Look into what the materials are for all Craft tools before purchasing, to understand whether you are in alignment with your own ethics if you purchase it.
The beauty of the internet is that we witches can literally buy anything we want from our phones and laptops, and that wasn't always the case. HOWEVER, when it comes to Craft tools that are very personal in nature, I still feel strongly about being able to pick it up before I purchase it. Some witches disagree with me, and say, "Until it's consecrated by me for my personal use, it's just a mundane object anyway." OK, if that's how you feel, that's how you feel.
I will tell you a little story about how my husband got his beautiful handmade Athame. It's gorgeous. They say that tools have a way of finding their witches, and I swear to you that this Athame called to my husband so that he would go from Minneapolis to Denver to get it. Here's what happened: Hubby wasn't with me yet, and he met a woman at a Pagan festival. There were sparks. She invited him to fly from Minneapolis to Denver to visit her so they could begin a romance. He was very excited. During his time with her there, they went to a local Pagan shop owned by some friends of hers. The Athame was in the display case. He couldn't see how much it was, but he kept staring at it as it called to him. The owner offered to let him hold it, and he picked it up. BLAM...he knew this was his Athame and he had to have it. He was nonplussed to find out that it was $700. It was worth every penny, but he didn't have $700 for an Athame. The owner offered to take payments and ship it to him when he had paid in full. The deal was done. He turned to the woman with whom he was visiting, and noticed, quite suddenly, that he was not at all attracted to her anymore. It was awkward. He had to tell her it wasn't working for him, and he felt bad about hurting her. Still, they had only slept together once, so there wasn't a massive emotional investment yet. He swears that his Athame brought him to Denver to purchase it. True story.
In general, don't pick up a tool unless you are prepared to purchase it. It can feel very bad to have a tool call you and not be able to have it. It has happened to me and I almost cried, it was such an emotional experience.
Stuff to think about:
Is the tool well-balanced?
Is the tool likely to be used by you often?
Is the tool well-made?
Is the tool likely to work well with tools you already have (If you have narrow chalices, a wide blade may not work well)?
Will you be able to properly care for this tool (does it require special storage bags or special cleaners)?
Does this tool speak to your heart?
Whatever you choose for your personal ceremonial blade, this decision is a big one. Many witches cherish their Athames more than any other possession. Take the purchase seriously, as this tool reflects you, your spirit, and your connection to the Divine. Happy searching!