What baby things shouldn't be second hand?

Many breastpumps (like the Medela Pump in Style) should not be used by more than one user, due to milk contaminating the unit. Some (like the Ameda Purely Yours), do not have the same issue with milk contamination. If you do buy a used pump, be sure to buy new accessories, especially the parts that come in contact with you or the milk, for sanitary purposes. Also, beware of used pump motors wearing out after use. Here is an article talking about used breast pumps.

Use caution with second hand cribs. If the slats are too far apart or if the construction is poor, they could pose a health threat to a child. Also if it is the crib your parents used with you, beware of lead in the paint.

I'll add toys to the list. Generally, used toys are fine, except when they aren't:

• If the toy is very old, it may not have been built according to recent safety standards.

• Please don't give away a broken toy if it's broken in such a way that could be potentially unsafe. If you're on the receiving end of a toy, inspect it before use.

• Toys that require batteries should either have batteries removed or replaced before giving away. If you're on the receiving end and don't know that batteries are fresh, best to replace them. Old batteries have a tendency to leak. Also, make sure the battery door and screws are still secure enough and not loose.

• Assume a toy is dirty. Clean it, but remember too that the toy may end up going in your child's mouth! – so be careful and don't clean it with anything harmful. Rinse well.

When in doubt, throw it out. Re-use is great, but be sure it's safe re-use.

I wouldn't use a second-hand crib mattress, if I could help it. Basically for the same reason I wouldn't buy a second-hand adult mattress. Bedbugs and such.

I recently saw someone here selling used cloth diapers on Craig's List. That kind of grosses me out too, but others may feel differently.

May be pushing the definition of 'baby' a bit far, but I'd include shoes in the list.

UPDATE Opinions seem to differ on this point. Here is a quote which is slightly more nuanced:

1. Baby shoes and young children's shoes are normally worn very little. Children's feet grow so fast, that these types of shoes usually show hardly any wear at all. These types of shoes are usually the most acceptable to pass down.

2. Make sure that hand-me-down shoes fit properly; never allow children to wear shoes they need to "grow into." This can cause foot and leg problems, not to mention being a tripping hazard.

3. Inspect the insides of the shoes for wear. If the insoles show much wear, especially, if you can see the imprints of the feet, toss them out!

4. Inspect the outsides of the shoes for wear. Are the soles in good shape, or do they have smooth spots? Are any sides of the soles worn down? Some children have a tendency to walk on the inside or outside of their shoes. If you see signs of this, toss the shoes instead of making them hand-me-downs.

5. Check your children's shoes sizes often. Are hand-me-down shoes damaging your children's feet? If they only get bigger shoes when a sibling outgrows a pair, then the answer to that question could be "Yes!"

There is some evidence that using a second hand mattress can increase the risk of SIDS. The theory is that toxic gas releasing fungus can build up in a mattress.