In searching google books I found only a handful of references to "land-sharks' but the one below I thought a good insight to a person who might engage in such behavior. Perhaps, you might find a land-shark in one of your books. Enjoy!
One class of persons against whom the new-comer must be on his guard is the "land-shark." There are land agents in Florida who are as trustworthy as the same class anywhere, and whose advice and assistance may be of great service to the settler; but, on the other hand, almost every locality is infested by one or more "sharks," who prey upon new-comers by offering them "the greatest bargain to be had in the State," the prices asked being usually about twice as much as the property could actually be bought for. Usually very plausible in manners and talk, these men are well calculated to impose upon the inexperienced, but a little inquiry among other parties will usually suffice to expose their true character. One rule should be inflexibly adhered to by the settler, and that is, never to be persuaded into "closing" hastily with a "bargain," and never to buy a piece of land until he has consulted two or three different parties as to its quality and price.
Source: Florida for Tourists, Invalids and Settlers ©1884