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Keep in mind your maintenance costs. Try to have your leases cover your maintenance costs, they usually do at 6-10% monthly fee (percentage of purchase price). Leases for one year or two are good, any longer and they will hurt maintenance.
Selling aircrafts through the request feature is a big income boost as you only pay 85% of the price to the manufacturer. That way you can give a slight discount (start with 2-4%) to airlines. Long time customers might get other discounts, it's up to you.
Now, you're playing as an airline too, so maintenance is less of an issue. This might let you lower prices more, just be careful not to flood the market with cheap aircraft.
If you have specific questions about the workings of the brokerage system just let me know in a PM
