1. Inspect the Piece, But Don't be Afraid of Flaws
When buying second hand, don't forget to give things a once over. This means picking it up (if possible), turning it upside down, looking at it from every angle, and opening the drawers and doors. If you do notice an imperfection, don't be alarmed. You are buying a piece that is old, which means that it probably will have some imperfections due to daily wear and tear.
2. Items I Always Consider
Furniture- Vintage furniture is typically very well made using good materials and excellent craftsmanship. Check and make sure the pieces are solid wood with quality touches like dovetail joints.
Accessories- For a relatively small investment, you can add a unique touch to any room with vintage accessories. For example, I have a collection of vintage globes that were purchased for no more than $10 each. They add a great pop of color to any room. Another type of vintage accessory that you might hunt for are radios. They don't have to be operational. If you like the color, shape, design, etc. it is a good buy.
Lighting- Vintage lighting should be rewired to adhere to current code. Remember to factor in the cost of an electrician as you may not be comfortable DIYing this.
Upholstered pieces- I will buy chairs or ottomans that I know I can recover myself. Chairs with fabric seats or removable cushions can be inexpensively refreshed with fabric and foam. But fully upholstered pieces often look best when recovered by a professional. And that is an additional cost to consider.