This requires a lot of explanation. You should visit there site and they will tell you the requirements of becoming a UL listed panel shop. http://www.ul.com/field/icp.html
There are basically two types of UL marks you should look for: UL and UR (backwards R then U). Just a UL mark means it is UL listed and does not have any additional requirements to be UL listed (UL mark). A backwards RU means it is UL Recognized. This means it should be tested when installed or there are additional requirements to be UL listed.(fusing may be required because the spacing within the unit is too close). For you, no matter what you select for components you will have to have UL bless your panels (at least at first). If you select UL listed or UL recognized parts then it will make the process easier and less costly. Visit the link above and it will explain everything. Good luck.

I forgot to mention that most standard components are not even UL recognized, ie resistors, capacitors, transistors, etc. But things like relays are.