If it was after the last patch (yesterday, I think) and if it was financed through a loan, that loan was immidiately repaid after you sold the plane, so only wahever you gained (or maybe lost) will appear in your bankaccount.
Anyway, this patch solves the cheating. People were buying planes on credit and then sold them without repaying the loan. The cash they received from selling one plane was enough to lease 10 more planes. So if you bought and sold several planes, a brand new airline could lease a fleet of 100 planes. Don’t know if they would be able to cope with the heavy loans/interests, but they were surely a nuisance.