The Seller Should be Reputable
You need to know you are getting quality wares, which is more likely to happen if you do business with a reputable seller. On the flip side, big names would almost always charge heavy premiums simply because of their own reputation.
Value over a Longer Term
The more monetarily valuable a piece is, the higher will be its investment potential, so consider that when buying a 2-carat cushion diamond ring. Additionally, there is less cost involved when you sell a more costly piece, similar to real estate.
Choosing colored diamonds can be a good way to get unique jewelry, but these would not be as easy to sell as colorless diamonds. You may want to stick to the more timeless pieces, holding the cut foremost in your mind. Trends of today can become dated tomorrow, so it may be better to stick to a classic style.