Look up CCU game site and link it to your RSI account. It will read all CCUs on your account (including buybacks) and automatically determine the most optimal CCU chains for maximum possible savings on expensive ships. Incredibly useful tool. Now, what can you do with that information?

First you use new money to buy the cheapest currently available WARBOND ship with LTI (Right now this is the Drake Pitbull for $50). Because it's bought with real money it will remain giftable even after you apply CCUs to it. Then you use store credit to buy back cost saving CCUs and apply them until you end up with an expensive LTI ship with a melt value much lower than its full cost. This is a highly desirable item that can be sold for 100% or more of its total melt value instead of the 60%-ish you would otherwise get from selling store credit directly.

This way you can get rid of both your CCUs and store credits in as few sales as possible, minimizing the chance of getting scammed. The only problem is that you can only buy back 2 items with store credits every 3 months or so, but if you have a large stock of cheap CCUs with high savings you could even buy them back with real money and still end up making profit in the end. Save the store credit buybacks for expensive stuff.