We present a detailed theoretical study of the recent proposal for selective nanomanipulation of nanometric particles above a substrate using near-field optical forces [P.C. Chaumet et al., Phys. Rev. Lett. 88, 123601 (2002)]. Evanescent light scattering at the apex of an apertureless near-field probe is used to create an optical trap. The position of the trap is controlled on a nanometric scale via the probe, and small objects can be selectively trapped and manipulated. We discuss the influence of the geometry of the particles and the probe on the efficiency of the trap. We also consider the influence of multiple scattering among the particles on the substrate and its effect on the robustness of the trap.