You don’t need your own webstore to offer Selling Assistance services, but you may want one anyway. If you already have a thriving webstore, great! That’s just another selling option to offer your clients. But if you don’t and you’re wondering if now’s the time to get one, I’d advise waiting.
Run your SA service for a while out of a marketplace environment such as eBay or Etsy. Then, if you later decide to upgrade, you’ll have a better sense of what your needs are from a webstore so you won’t go into the process naive. You may even discover that a marketplace is a better platform for your needs anyway, and then you can just save yourself the money and trouble of setting up a standalone webstore.
Many marketplaces also give you a storefront for free as part of your account, though some, such as eBay, only do so for a fee. These storefronts have a lot of advantages, such as giving you a simple and consolidated way to display all your items at once, custom category names and pages and, in some cases, advanced tools like email marketing and cross-promotional widgets. Even a storefront that isn’t free, such as eBay Stores, can be worth it if the discounts and perks of ownership offset the upgrade fee.
There are a lot of reasons why it’s a good idea to start selling on your own website instead of on a platform and a myriad of choices of how to do it. It’s also the sole topic of one of my other books, Beyond Amazon, eBay and Etsy: free and low cost alternative marketplaces, shopping cart solutions and e-commerce storefronts, so I’m not going to repeat myself here. But understand that it’s (usually) much more work over selling on a platform, so it’s likely better for you to grow into rather than try to use right from the start.