Condition
According to your needs and condition of your bike, you will need to make a big first step, and choose what type of part or accessory you need to get. Buying vital parts will be more expensive and you won't be able to distinguish one from the other, as they're virtually the same as a stock part. Such motorcycle parts accessories and other vital additions need to be acquired from an OEM (original equipment manufacturer). If you are in for a new seat or cover then you can even get an aftermarket or a used one - just try to find a good balance between a cheap price and a solid quality.
Customer Service
I cannot stress how much attention you need to pay on customer service. Because if a company is frequently helping their customers with product related problems, that means that they value your decision and you as a customer. A good tip here would be to check if a company's customer service is a top-notch one, and this is best done by finding out what are the ways to contact them, and if a company has many of them on display, you're in the right place.
Price
Talking about price, surely, you'd want to avoid overpaying for motorcycle parts accessories and other additional gadgets, but that doesn't mean you have to bargain for every single price out there. According to the price range of any given product, you should do a little bit of research whether or not its in the same price scope elsewhere and if it's offered in the same condition as well. Making a purchase right away can be a costly one as one should know that there's almost always something better in store (or online) for your bike.
Returns
Advice / Support
When problems such as installation or part positioning arise, there should be someone you can ask for help. The company or sellers store or website should provide you with written instructions or with a help of either the store worker or from a 24/7 tech support right on their website. If you are not satisfied with their service you can always make a complaint and return their product for good - just try not to use this to meddle with them.