I have been a Sellware user since 2015 and I can't say enough good about it. With 10+ years of professional eBay experience, I have tried over 10 different listing programs (mostly saas) and when it comes down to customizability and doing bulk listings nothing compares. With the help of Sellware, we went from a couple thousand eBay listings to over 30,000 and the power to manage them.
My favorite parts about Sellware are the eBay listing templates and amount of data you can upload in a CSV.
Lastly, I would like to say they have outstanding customer support. The guys on the other end are the same guys doing the work on the back end so they always have a fix or an answer to an issue.