There actually is a rule in place that prevents teams from doing just that. Found this on ESPN:
"The NFL doesn't want teams purchasing assets in trades. In a worst-case scenario, it would spur free-spending owners to buy up players, draft picks and cap space without giving anything but money in return. That approach would counter the league's revenue-sharing and equalizing conceit. The policy states: "Any trade of a draft choice or draft choices, or of player contract rights or negotiation rights, for any amount of money, is conduct detrimental to the league."