REVIEWS:
Keep in mind that most online stores carry a variety of products from a variety of brands. So although the company may have phenomenal reviews doesn't guarantee that all of their products will be of great quality. Be sure to check what other customers says about the quality of each piece, prior to purchasing. You don't want to order a product that looks good on a photo, but in person feels/looks terrible. Some online stores will sell sample products, or send fabrics, for free or for a small fee to see the quality of it up front. You also want to see what the terms and conditions are for companies with their return policies. You want someone that has a fairly open return policy in place, just in case you or your client hate what ends up being delivered.
PRICING:
When you find a product, don't purchase it from the first site you see. Use the name to see if you can find it for sale on another site. You want to make sure it's the same exact product, and not a cheap mock-up, but some sites will have the same exact product for cheaper. For example, Wayfair and Joss & Main are one and the same, yet their pricing on products on both of their websites vary significantly. Do your due diligence and you may save yourself hundreds of dollars, if not thousands. Those savings can go back into your pocket or be passed onto your clients.
SIZING:
Be sure to review the sizing dimensions of each product and ensure it fits into the space you will be using it for prior to purchasing it. Double checking the dimensions online can save you the headache of having to return the wrong sized product in exchange for another.