There is actually a "Dollfie & BJDs" section within the Dolls by "Brand, Company and Character" section on Ebay. Then within that it's separated between "Mini, MSD", "Full Size, SD", "Tiny, Anthro" and "Other". However, due to the options sellers have when creating auctions, you can find Tinies in SD, MSDs in Other, etc. and you'll have to weed through tons of doll accessories so your best bet might be to refine your search from highest price to lowest (or vise-versa). As GH mentioned, you can use the "-" (minus) symbol to help remove unwanted items from your search. You can even remove several by just adding a comma between (i.e., Luts SD -shoes, -wigs, -eyes). Also, try mixing it up with company names that people might get wrong - Doll Heart instead of Dollheart, Lutz instead of Luts, Doll Zone instead of Dollzone, etc. Also, some lines of dolls are popular on their own and you can try searching for them specifically, such as Minifee instead of just Fairyland or Delf instead of just Luts.
The most important thing is - be wary when dealing with Ebay. I find at least one recaster there a week. Recasters aren't just thieves (which is bad enough), but the dolls they create are not only inferior products, they can actually be harmful because of the chemicals and compounds used to make them.
While there are several reputable dealers who represent specific manufacturers (like Junky Spot, J-Pop, Doll's Singing, etc.) there are others that exist strictly to rip you off. Pay careful attention to the negative and neutral feedback. Personally, I won't deal with someone that has less than a 99% rating and I look very carefully at any seller with less than 99.5%. And if you have any questions about any of the sellers on Ebay, don't hesitate to ask us our thoughts.
To be honest, for your first doll (or two), going directly through a dealer or manufacturer is the safest bet. Many dealers and manufacturers offer layaway and when you consider that in the BJD resell market a "used" doll normally goes for as much and, sometimes, more than the doll original cost and the private seller often doesn't offer layway options it can be the better route. Denver Doll and Mint on Card offer layaway options, for example as does Iplehouse, off the top of my head.
Sadly, Junky Spot does not offer layaway anymore (but Emory has his reasons), so if its an Obitsu that you have your heart set on, the secondary market will be your best bet.