Even nowadays, most people are honest. Your best bet is to stand by what you sell. The returns policy is a selling point for many, so be careful with how you word your policy. You should always operate under the assumption that the system will not be abused.

With that, there's no way to completely avoid others taking advantage of you. This is part of doing business. Do it right and the advantages (increased conversions, AOVs and customer loyalty) will far outweigh the bad.