The best place to buy loose diamonds is from a diamond dealer or diamond jeweler. This means that you need to be sure the dealer you are going to buy from is qualified to sell diamonds and not just an outlet to sell whatever they can.
Here is a checklist one can use to pick the best dealer to buy loose diamonds from.
Firstly can the jeweler or dealer supply a FTC regulated Lab certificate with the diamond? Can they provide a GIA appraisal with the diamond also? Do not even entertain a dealer that does not have any certificates and appraisals for their diamonds.
Do they have a master set of diamonds for color grading? To grade the color of a diamond you really need to compare it with diamonds of an established color. Do they have a gem diamond light for color grading? Do they have an ultraviolet light for color grading? Unless they can demonstrate the resources and equipment for a proper color grading then it is unlikely they will be able to color grade accurately any diamond.
Any decent diamond dealer should have the GIA scale for grading color and clarity. If they do not, go elsewhere.
The diamonds should be loose of course. Do not be convinced in to buying diamonds that are set when you are after loose diamonds. You can bet they are of a lesser quality and flaws are likely to be hidden. It is not possible to grade a diamond properly when set either.
Do they have a trade in policy whereby you get equal to what you originally paid (not including taxes and shipping? If not then it is likely they will not trade in and return your funds. This is important when you buy online as you really have no way of knowing if the diamonds you are buying are suitable until you get them.
Is the dealer American Gem Society (or equivalent in your country) rated? If not then go else where.
Do they have GIA staff on the premises. If not it is unlikely they will be able to rate gems correctly.
If the answer is yes to all questions them you should be fine to buy diamonds from that dealer provided you follow some basic common sense and practice a little due diligence. That way you can be sure you will find the best place to buy loose diamonds