Note, there is a very helpful left search bar. If it disappears, click on the arrow to get it back.

There is a sub category on E-Bay: "Vintage Cycling".
Vintage Cycling | eBay

I don't like it since there is a lot of overlap between new and vintage bikes, parts, and frames.

For frames, for example, there are sub-categories for frames.
Bicycle Frames | eBay
Vintage Bicycle Frames | eBay

Once in the sub-category, you can specify features like frame materials or sizes.

I tend to do a lot of searching. The trick is to pick out your favorite search terms.

Basic E-Bay LOGIC:

• For "AND", simply list things items on a line with spaces, or use parentheses.
  
  steel Raleigh
  (steel Raleigh)
• For "OR", add commas between search terms, generally in a parenthesis.
  
  (Peugeot,Raleigh,Mercier)
• For "NOT", put a space followed by a minus.
  
  campagnolo record -carbon -ergo -ergopower
• You can combine features using a combination of parenthesis and spaces and commas.
  This should get you Shimano or Suntour Road Shifters that are not carbon, and not brake levers or "brifters"
  (shimano,suntour) road shifter -carbon -brifter -brake

It should be obvious there is a certain amount of voodoo in E-Bay searching. Make your search too inclusive, and you're wading through a lot of unnecessary trash.
Make it too restrictive, and you miss the super deals.

I like the "similar items" which often pick out items that my searches would otherwise have missed, or I would have overlooked.

I frequently sort search results by price.

If you're picking out something like a bike light and getting a lot of clutter, you can also restrict to either "Auctions" or "Buy-It-Now", depending on whether it is something that you might expect lots of fixed price items.