To make the most, it is worth narrowing down your focus so that you gain in-depth knowledge, rather than trying to cover everything.
Choose one country to focus on, then research which coins have low mintage, errors, etc. Check out more forums, keep an eye on prices on ebay, and look at websites dedicated to the country you are interested in.
For a starting point, you might want to go to https://worldcoinsinfo.com/world/ru...e-coins.html
and get an idea of what different countries coins are worth.
If selling on eBay, with a free account your listings are limited, so go through and End Listing on your low value coins to replace them with higher value ones. If you plan to do a lot of selling through eBay, get the Basic Seller Account (monthly subscription) so that you get 600 listings per month + reduced final value fees.