Hi Dale -
Selling on eBay is awesome from their mobile app! You can easily start a listing by scanning most UPC barcodes or by completing a quick search. Once your item info is populated (or after you manually enter it), the eBay app has a great background removal tool that can be used on Android, iPhone, iPad, tablet, etc. Here's a quick video of how to use it: Link hidden: Login to view
Mobile Security: Use two factor authentication when available, strong passwords, don't follow random links :) I only have experience with Webroot's Mobile Security. I have had no mobile issues. Accounts get hacked quite often so good question. I personally use a password manager (LastPass) b/c I am terrible with passwords. That's something that you will want to weigh the pros and cons on. Basically, you need to consider the tradeoffs between productivity and security. With a password manager, they can greatly increase your speed but it is a single point of failure type of deal (depending on how many passwords you are using with it). For that, I use a blended approach where my most critical passwords are not stored in the pw mng but everything else is.
Finally, the most beneficial apps for me have been those that cross promote. This greatly reduces the time spent creating multiple listings on multiple platforms. My model includes several platforms so having the ability to make one listing and add it to 5 marketplaces is a YUGE win! I am currently using listperfectly and I have a shopify website that I use as an integration tool. Link hidden: Login to view On the sourcing side, I look at trending items on Ali, Amazon, etc. Mostly this is for getting a ball park on margins, sales volume, and availability. When doing retail arbitrage, I use QR Reader to quickly scan UPC barcodes to see if there is $$ to be made!
Hope that helps and good luck, sir!