If you aren’t selling anywhere yet or have only limited experience, you should really get some practice selling before you begin. Clients are trusting you with their items and potential profits because you’re presenting yourself as an experienced seller. If you’re really just starting out yourself, you’re going to be making mistakes that could damage your reputation before you even get started! Even if you’re a veteran seller, selling items for someone else can be very different from selling your own items, and you want to make sure you understand what you’re in for before you dive in.
A simple way to practice at being a SA before actually starting your service is to buy a large, mixed lot of items of the type you’d like to sell with your service either at a garage sale or on a site like eBay. Let’s say that I’m interested in specializing in action figures, but I want some more selling practice before I jump into taking on clients. I find a mixed lot of over 100 random action figures on eBay, and I buy it somewhat blind. Then I proceed to go through the lot I purchased as if it were from a client, organizing, cleaning, researching, photographing and listing each item the way that will yield the most profit. Actually going through this process will give you an idea of the questions you’ll need to be asking yourself on each contract (“Would the Avengers figures sell for more if I list them each individually or should I list them as a lot because that’s faster?”), and what kind of things you should see if the client can answer to save you time (“Which weapons go with which figure?”).
Keep careful track of how long the entire process takes. Once all the items have sold, make up an itemized spreadsheet of all the fees and profits as if you were giving it to a client as a receipt. You may not make a profit on this practice lot, but that’s OK, as it’s a business expense and a worthwhile investment to get a feel for the SA life in a practice environment. It’s well worth a little loss to get some hands-on experience to help you anticipate issues and get a sense of how long different steps in the process are going to take.
Just a word of warning. You’ll get better at this as you go. Your first time selling someone else’s items will take much longer than it will in the future. You’ll need to do less research with each item you list, the work will go faster as you develop a system and, most importantly, you’ll develop the sense to only take on items that will be worth it to list, something that may not be the case with your lot of practice items. So, while practice is a good idea, don’t assume you’d be a terrible SA if your practice goes poorly.