I will usually purchase perfumes that aren't completely full, also, if they are collectible or sought after.
I look for:
1. Current designer brands (Chanel, Burberry, Armani, Dolce & Gabbana....)
2. Discontinued fragrances (Almost any discontinued perfume has a disappointed fan somewhere. Some older Mary Kay does well in this category.)
3. Older perfume bottles are very collectible, even if they are empty. These tend to be bottles from higher-end fragrances. Look for heavy, well-made bottles, unusual designs or figural bottles and anything marked France. Some very collectible bottles were made by Lalique, Tiffany, Baccarat, and Steuben.
I'm often asked about Avon bottles. Avon made many hundreds of styles of bottles, most labeled as collectible. I have found that most don't bring much money on Ebay at all. The exceptions to this are the newer high-end fragrances that Avon has been producing recently and some fragrances that were discontinued.
I have found perfumes to resell at garage sales, estate sales (check the bathrooms and dresser tops), thrift stores, auctions, and overstock stores. There is a great market out there for collectors and just people searching for their discontinued favorites.