As GeoffHa has written, many (the majority) are cheap and can be found in good condition.
The ones you should be careful with are overprints in general and from several countries. There are many fake overprints. The more expensive overprints are being forged. Also, those sets that are expensive, usually, are so because they are rare in good condition. Those are the ones you have to be careful with,
If you buy an expensive set without a certificate, be sure they are genuine and with original gum (if unmounted or unused).
There are many reputable dealers in Germany but there also are Italian dealers that offer stamps overprinted like those from Yugoslavia. Some auction houses regularly auction the better items. Delcampe,
ebay, and Hipstamp are good enough for most stamps.