I am of the opinion that if you have to ask whether it's ethical, then it's probably not. But here goes anyway...

On the Web, I recently purchased a set of plans to build a "public domain" illusion (and no, I don't mean a de facto public domain illusion like Zig Zag). These particular plans aren't exactly groundbreaking, but the creator has incorporated a few unique touches that make his version a bit better. I have about a day's worth of work left to completion, and I discovered that the plan set I bought was an illegitimate copy. The plans are long out of print and, although they aren't particularly collectible, I haven't been able to find another set for sale.

I have contacted the creator, and he is searching for an original set that might have been packed away. If he finds one, he will send it to me.

Would you:

a) Stop building until you receive the legitimate plans?
b) Complete construction and not use it until you get the legitimate plans?
c) Complete construction and use it, and say, "Well, I tried..."?
d) Do something else?

If the creator says "go ahead", then what's the problem?

He hasn't exactly said "go ahead"...but only that he will look for a set of plans for me. He didn't seem too worried though, since he hasn't even looked at the plans in ten years or so.

Ask him.

If it was me (as the creator), then I'd give you a prompt and honest reply.

My guess is that, as he is looking for the plans for you, then you'll be OK. But ask anyway, you'll sleep better.

I think you are tying yourself in knots unnecessarily. Personally, I'd just build it anyway if I'd paid for some plans.

But if you are that ethically challenged about it, and were prepared to pay for another set of legit plans, why don't you simply forward the cash to the creator and continue building off the bootleg plans? Then if a set of genuine plans turns up, you are happy. If they don't, you are still happy. You paid the creator, and you still got to build your box. If you paid him, he might even check over your bootleg plans for you to see if there were any omissions.