i imported my bus myself, and my mate packed it with stuff i bought too. It is a lengthy process and a bit butt clenching, but as long as you have good communication with whoever helps you in the states you should be fine. You'll always be taking a risk buying anything you havent seen, but you will be saving time and money buying one that isnt rotten!!
importing my bus was the best thing i ever did, i could no way afford uk prices for either a uk bus or one imported by a company over here. My bus would of easy of had a mental price tag on it, and i paid peanuts.
as long as you use your common sense and everything feels right, it will be a really exciting experience. i couldnt wait for my bus to get here, it was on a ship called "the stuttgart express" it was nearly 4 weeked on the water, and just a few days in customs. the ship was owned by harpag lloyd, and they have a really good tracking system that lets you see where your boat is every hour on a map, i was looking a few times a day!!
Also, my advice would be, if possible, dont buy on the samba, there are some awesome busses on there, but i found that they all carried a bit of a premium, and it was a con artists dream on there, some of the best bargain buses are on places like craigs list, mine wasnt even for sale and hadnt seen the light of day in 16 years, but my mate convinced the owner that he should sell!!
Good luck mate, and i hope you get a really good bus.
nearly forgot, use the shippers "kingstown shipping" they are in hull and will help yo get it over here. nice people. dont waste time thinking you will make a saving booking a shippers in the usa, there are more little charges than you think. Kingstown will give it to you straight.