Staggering growth levels in the number of mobile devices and amount of mobile Internet usage has caused network providers to move away from unlimited data plans to less flexible charging models. As a result, users are being required to pay more for short accesses or underutilize a longer-term data plan. In this paper, we propose CrowdMAC, a crowdsourcing approach in which mobile users create a marketplace for mobile Internet access. Mobile users with residue capacity in their data plans share their access with other nearby mobile users for a small fee. CrowdMAC is implemented as a middleware framework with incentive-based mechanisms for admission control, service selection, and mobility management. CrowdMAC is implemented and evaluated on a testbed of Android phones and in the well known Qualnet simulator. Our evaluation results show that CrowdMAC: (i) effectively exercises the trade-off between revenue and transfer delay, (ii) adequately satisfies user-specified (delay) quality levels, and (iii) properly adapts to device mobility and achieves performance very close to the ideal case (upper bound).