You've got different things going on. The exemption is described at http://www.jacustoms.gov.jm/home_tem...ors&group_id=1 but I'll paste it here:

The Customs Tariff allows for each passenger to receive duty concessions in respect of the following:
  • Instruments and tools to be used for the purpose of their profession, trade, occupation or employment, provided that the items have been in the passengers’ possession and bonafide use for a reasonable period;
  • Tobacco and some potable articles including wines and spirits in the baggage or on the person which they might reasonably be expected to carry with them for personal use, in such quantities as the Commissioner may from time to time approve;
  • Used household effects which have been in the passenger’s use for a reasonable period of time and are not imported for sale;
  • A passenger who is eighteen (18) years or older and who is the holder of a Jamaican passport or is ordinarily resident in Jamaica is entitled to additional duty concession in respect of other personal or household effects not specified in (1), (2) or (3) above which the proper officer is satisfied are not imported for sale or commercial exchange and do not exceed US$500 in total value. This concession will not be granted to a person who has been permitted duty free importation under this section within the six-month period immediately preceding the date of return to Jamaica.
So there is no exemption for anything intended for sale or commercial exchange. In general, gifts are treated as commercial exchange (otherwise everything would be "I give you a gift of a TV set, and surprise! you give me a gift of cash!").

Once you're speaking with an actual customs agent, she needs to make a determination as to whether the items are your household effects that you've owned for a reasonable period of time. Someone might get lucky with a "new in box" monitor, but clearly that can't be relied on. Whether it arrived as your luggage on a plane or in a barrel of stuff you shipped commercially doesn't matter to customs.

The customs laws are so complex that the determination of an inspector on a given day will vary. This is true at all borders; I used to regularly carry merchandise across the US/Canada border (both ways) and had a variety of experiences, including once being completely turned back and then going to a different border crossing where the result was better.

Courtesy to customs officials always goes a long way. I'm often astounded to watch tourist getting into it with the officials insisting that, for one example, their travel agent told them that something was OK so the customs official had to accept that.