Well, you have done everything up to this point, and have done a listing for your products you wish to sell. Do not be discouraged after this if it takes a while to get a sale, maybe you will have a sale right away, and sometimes you will make more than one sale too. It varies for everyone, and consistency is not something that comes with selling online.
Ok, you made a sale, and with most market places an invoice has been sent to the customer. Everyone does everything differently, and again you will have to find the way that works for you, and feels comfortable for you too. Once a sale is made I send a personal note to the customer thanking them for their purchase, and asking them if there is any change to the mailing address to please notify me of it as well so that I may note it.
Once payment is made you will then have to prepare the item for shipping, or sometimes you can have most of the process done while waiting for payment too. Make sure you have supplies on hand for the method of shipping you have chosen. Boxes, mailing tape, bubble wrap for delicate items, peanuts, or whatever supplies you choose to use for your shipping. You want to do the best you can wrapping the item to ship as once it leaves your presence you will have no control of how it is handled from then until the customer receives the item they have purchased.
Another step to follow through with after receiving your payment is to prepare the shipping label, and also I recommend enclosing an invoice with each order you process. I myself enclose an invoice with the item for verification for the customer, and I make a copy of the invoice for my records as well. You will also want to consider enclosing a business card, maybe a thank you note, and some sellers include some small gift along with every purchase of maybe a pen, a magnet, or whatever is used to represent you, and your name. No rules for any of this, and it is your decision of what you will, and will not do.
Ok, you have securely wrapped the item, properly boxed it for shipment, and it has been delivered to the post office, or a pick up was done of the item for delivery. After this processing is completed it is always curteous for you to then notify the customer by thanking them again for their purchase, and that their shipment has been made. This will better help them to watch for the item, and kind of when to expect delivery according to the shipping service used.
Keep in mind, you do not have to do all of these extra steps, and there are plenty of sellers that have their own ways, do just a few things, or really do nothing at all except to package and ship. It all relies on the customer service that you wish to extend to the customers, and how you will be remembered for your services too.
I know there is much more that could have been covered, and it could have become very lengthy too. Yeah, more winded than I have been:) Most of what has all been covered with these topics for selling is the basics of doing things for Online Selling. I hope this has helped you with things you do, for those new to online selling, and maybe a bit of a reminder for some as well too. I wish you all well, and best of luck to you with all you do. Take care, have fun, and enjoy your new venture!:)