I have ordered from Mike's Wet Pets (off of aquabid since I don't buy as much off of Ebay) often and really like his plants. I have never had any issues with his plants. His bunched plants have a nice amount of individual plants to each bunch and his single rooted plants have very good root systems.
I have also bought from plantedaquariumscentral.com. I found a lot of plants that I couldn't find anywhere else on there and the plants came in pretty good shape. No complaints and I would more than likely order from them again in the future.
My favorite here lately is www.bamaplants.com. His sn on aquabid is NoFishToday. I have ordered tons of plants from him within the last couple of months off of aquabid and from his site. His plants are always very nice and I have not had any transition issues from any of the plants.
I also prefer to buy from other hobbiests when they trim their tanks because it helps them out plus you normally get nice and healthy plants.
With hitchhikers--most plants (unless otherwise specified) are a normal part of buying plants. Taking the plants and dipping/swishing them in a light bleach water solution will kill off any hitchhiking snails or other possible disease. I have gotten snails, tiny shrimp, and know of people who have gotten fish fry/eggs as hitchhikers.
I've been looking at ordering a couple snail (they aren't available in my lps), from what I've read assassin and nirite(sp?) snails won't reproduce in my fresh water tanks. When I asked at the LPS they laughed and said that they would reproduce like any other snail & I'd soon be over run. Which is true? Do your assassin snails reproduce in you tanks as well? I already have several mystery snails, so I was very interested in some assassins.
Well I found a baby assassin snail today in my freshwater tank today so he is wrong at least in that regard :D
I ordered some plants from aquariumplants.com, and my main issue with them is that they ship only once a week, and you have to order by Sunday to make the next week's shipment. Therefore, my "two-day" shipping took me two weeks to get.
The plants were fine, but the plants from Planted Aquariums Central, above, look healthier than mine did when they arrived. I will try them next time I order.