I'm having a hard time differentiating the verbs "to retail" and "to sell". What's the difference between those two?
To retail something is to sell it in a specific way.
These are from Merriam-Webster:
1 : to sell in small quantities directly to the ultimate consumer
: to sell (something) in quantity usually for resale
In both, something is being sold. The difference is in the quantity sold and to whom it is being sold.
In short, when you sell something it can refer to either a retail or wholesale method of sale.
As mentioned in a comment to the question, sell can be used figuratively, in a sense inappropriate to either retail or wholesale.
Sell is also often the more idiomatic word. Except in particular contexts, it would be strange to hear one of the other words used.