Often times, when doing business with "middle men" type of companies, sales (i.e., more revenue) is the driving factor behind business practices and quality control is neglected or forgotten. Remember, sites like these earn a commission on each sale they generate for that hotel/resort.
Do your homework and ask for references from the company and ask for the name of the hotel/resort so that you can look up unbiased reviews for that hotel/resort on sites like Trip Advisor, etc.
The Caribbean is approaching its low season, so this may be one big reason for specials, but when it's just too good to be true, always remember to do your homework. Often times you may find that the location and or the hotel/resort is just no good.
Regards,
Mr. D. Bennoit