In this sentence:
The salesman would have sold his products but he was late for the appointment.
1- What is the meaning of would have sold in that please?
It means that, in a universe where he was on time, the customer bought the products. The products were something the customer wanted. The sale was a sure thing -- except the salesman wasn't there on time, so the customer left, or got angry enough not to want to buy anything, or bought someone else's products.
2- What is the meaning of would have alone in that please?
It means that a thing was expected to happen, if matters had gone that way. "I would have helped you, if I had been home." "She would have been dead, if she had not been wearing her seatbelt." "He would have lived, if he'd worn his seatbelt."
3- If you replace the words would have sold with some identical other three words, could you do that? If so, what words would you use please?
In that sentence, I cannot think of any other words you could use that would retain the meaning of "If X, then Y, but X didn't happen." You would need to re-structure the sentence entirely and drastically, such as: "In a perfect world, he got there on time and the customer bought all his products -- but this is not a perfect world, and he was late, and the customer had gone."
You could substitute "could" for "would," which gives a slight bit of uncertainty to that alternate outcome: "he could have sold his products" suggests that he had a good chance of doing so, but it was not the almost certain outcome that "would have sold" suggests. For even less certainty, you could say "he might have sold his products."
Of course, as you know the answerer just can't tell you the bare meaning for these kind of modals; so, my approach to answer would be setting the meaning of the sentence straight.
The salesman would have sold his products but he was late for the appointment.
Mr.X is a salesman. He was probably unsuccessful in selling his products until, Y offers a great deal of money for his products. Unfortunately, since he has a significant appointment with, I don't know, maybe his dentist; Mr.X has to reluctantly decline the offer. This is what one can fathom of the meaning of the sentence. Now, let's try to get these meaning from the sentence.
First, let's think "the second condition (his setting of appointment) was untrue". The rest of the story would be "the salesman definitely makes the deal and sells his products." (this sentence is stage 1) To state these two sentences combined (the ones in quotations) together gives us this:
{If the setting of the appointment was untrue, Mr.X certainly made the deal.}(stage 2)
The conditional sentence above, is the opposite of what the main sentence tells us. So, to make it apply what we need, we use "would have" + p.p. of sell to say that something didn't let selling happen, but if it wasn't for that thing, the selling definitely happened. This is simply step-to-step addition of meaning.
{Unfortunately, Mr.X has an appointment. He definitely wants to sell his products, but now he can't.} (stage 3) Which turns into "He/would have sold/but/has/appointment". Finally the main statement takes its figure.
And as a replacement, I think because it's almost about "definite" talk in here, and the probability isn't mentioned, there are hardly any 'three identical words' which would completely 'fit' in the sentence instead of these three. If one asked me to write a statement with the same meaning, I would prefer doing a recast.
