when buying on ebay spend some time comparing sellers, prices, delivery ect...
when you have found the item you want, make sure you pay immediately as then you will get great feedback and recieve your item quicker.
i tend to go for the buy it now items, depending on the time left in the auction and bidder amount, set yourself a bid limit and set up autobid to that amount, saves having to be watching all the time.
always leave feedback and if the seller has been very helpful and very satifactory, add them to your favourites so you can see what other items they are selling.
when selling take even more time, make sure your description is very detailed, explain everything you can and know about your item, make sure that you put in model, brand ect in the part you can, makes your item show up more when people search any part of what you have put, also make your headline eye catching and not just a few words, make it what people will type for a search so yours shows up.
will update this another time when im not busy