There is a basic list in the FAQ section that I can't seem to find at the moment (just got up...) that lists what you can and cannot import. But also take into account that whether it gets seized or not has little to do with the actual laws and lots to do with what that specific inspector thinks.
For example, non-prohibited length pistol slides and barrels, M203 shells, and mags with dummy rounds visible (TM G36 lowcaps for example) are NOT against the rules but OFTEN get dinged because lots of customs inspectors think they are against the rules. Then you need to go through a long appeal process if you don't want them to just destroy it. Mechboxes have also been seized from time to time, there is always a risk regardless.
Things like whole guns, receivers, and things that are really obviously restricted/prohibited gun parts are illegal.
Things like solid colour mags, internal components, inner/outer barrels, RIS/RAS units, etc. are all okay 99% of the time.
Also remember if you order from the States they have several EXPORT banns. Like NVG's, RIS/RAS units, body armour, anything made by Magpul, etc. etc.