The Trade Office is a free market. If you put something up for sale, you're required to determine an agreed offer/cost for your product. As soon as you post the product onto the Trade Office, you are agreeing to the terms of the sale set by you. Any player, whether they're new or experienced is free to complete the transaction for your product so long as they meet your offer.
While it's very noble that there are some players willing to put up cheap trades for new players to take advantage of, there is absolutely nothing ethically nor morally wrong about someone completing the trade in an effort to resell the product at its proper market value. There are some players who engage in this daily as a source of income (otherwise known as "playing the market", and it's not against the rules.
If you would like to trade items or resources to a specific person, then you're advised to do it privately. If you continue to put products up on the Trade Office that are intended for a specific player, you are doing it at your own risk.