Take good photos.
Take close-up photos of the "selling points" of the bikes eg: lugs, embossed details.
Women's vintage mixte bikes will sell for $100 or so; throw a $15 basket and a $3 bell and you can get $200.
Put a $3 bell on any bike you sell and mention it in the ad, even better. mention it in the heading.
Mention the size of the rider, not the bike, very few people will know the size they need.
A good example of a CL heading: "Vintage ladies bike, suits a rider 5'3-5'9, pink, bell, fenders, basket"