I suppose it would only be unethical if you had no intention at all of trading for Garza and some GM gave up their WW position solely on the expectation of being able to trade him to you. However, you didn't make this deal at all with any one GM specifically - and you said you would be "interested" - not that you were offering x player for him.
Can't really say thumbs down - if anything, a GM who saw your post should contact you first before making the waiver claim on Garza to work out a tentative deal with you once the claim is successful - and then if you bail out on him after he does it - then

If they go ahead without talking to you first and then find that your "interest" is only mild, they've assumed the risk I think.