The Collaborative International Dictionary of English v.0.48:

Undersell \Un`der*sell"\, v. t.
   To sell the same articles at a lower price than; to sell
   cheaper than.
   [1913 Webster]

WordNet (r) 3.0 (2006):

undersell
    v 1: sell cheaper than one's competition [syn: undersell,
         undercut]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "undersell":
   close out, convert into cash, cut under, dump, effect a sale, job,
   make a sale, market, merchandise, move, resell, retail, sacrifice,
   sell, sell off, sell on consignment, sell out, sell over,
   sell retail, sell short, sell up, sell wholesale, turn into money,
   turn over, undercut, unload, wholesale