An attractive proverb
If do you want pears, why do you buy apples? What is the difference, please?
"if do you" does not sound like a standard English conditional.
To add a bit to ion1122's answer, "if you DO want" can be used for emphasis. But that wouldn't be a good choice here, because it would suggest some context that we don't have.
Standard is "if you want". OK (but seldom used) is "if you do want". But "if do you want" is not grammatical English.
if you do want = if you in fact really want
Even better: why do you buy? (-ing is hard to pronounce clearly)
That is and has always been accepted.
What is the difference between jestli and když? Are they interchangeable?
Often they are. For example here. But in other sentences they may not be.
In sentences where if and when are not interchangable you will use když for when and jestli, pokud, zda for if.