That's not as bad as actually trading all of one of your resources away for other things, and then using "Monopoly" to get it all back. I've seen people play that way before.
Underhanded, yes. Illegal, no. The problem with playing this way is that it can really poison the friendly feel of the game. I had a player who used this tactic, as well as other aggressive ones like extorting good trades from people by threatening them with the robber. When someone plays like that regularly, the best way to deal with it is for others to gang up on them, which then leads to arguments about people being out to get them, and grudges held between games. It wound up being so bad that I stopped playing Settlers if this other player was going to be there.
As far as just using trade offers to find out what people have, that's not nearly so bad. Heck, you don't even need to do that; you can keep track of what resources everyone has. The only way someone might get resources that you don't know about are with the robber, but that leaves the same set of resources on the table, just in a different person's hand. So, you shouldn't even have to do this; just keep better track of what resources people earn and what they spend. If you can't remember, just try and think back for what the last few numbers rolled were, and what the last few things they built; you should be able to work out pretty well what they have a lot of.
If you offer trades to people, and then just grab the resources with "Monopoly" instead, they'll think that's cheap and feel burned; you've gotten their hopes up for a trade, and then dashed them by stealing resources. If you just keep track of what resources they have, and then seem to "magically" guess the right ones, they'll be disappointed that they lost the resources, but impressed that you managed to get so lucky with your "guess." How would you rather people feel about you?