There are popular online stores for Transformers like
Big Bad Toy Store and
TFSource. There are many others but these are the big ones. The prices, however, are going to be somewhat high and the inventory may not be that extensive. You won't find MP-01/Optimus Prime but you'll find other figures like MP-02/Ultra Magnus, MP-04/Optimus Prime with Trailer, MP-05/Megatron, etc.
If you do want to try eBay, I wouldn't be concerned about the location of a seller on eBay. It's really more about their feedback score. I personally look at the score as well as how many negative, if any, feedback they have. I've purchased from several Chinese sellers on eBay with no problems. They all had 99.5% or more for feedback scores, which is not necessarily a great thing by itself. Sellers can have thousands of sales so half a per cent can mean quite a few negative feedbacks. I checked all of their negative ratings and saw that often the issues were relatively minor(such as scuffed/dented package, which I'm not concerned about as I'm not an MISB collector).
So I suggest checking the score, how many negative feedbacks there are, and also the nature of the negative feedback.
Be sure to check the prices and types of items sold previously by the seller. If the seller previously sold $3 items and now is selling a few $50 products all of a sudden, I'd err on the side of caution. If I see that a seller has regularly sold higher priced items, especially if they are of the type I wish to bid on or buy, I know that at least the seller has a history of dealing in this kind of item.
Most sellers should have personal photos of their items. If they have stock photos, it's another sign to be wary of.