eBay rules are that if you bid on an item you are under contract to buy it if you win. If there is a valid reason you can't complete the purchase you have to ask the seller to cancel the sale and give the reason.There are only a few reasons you can cancel a sale, you will be asked to choose the reason from a list supplied by eBay.
If you are selling an item on another site as well as eBay you need to state that the item is also for sale on another site and that it can be withdrawn at any time. I would not bid on an item if I knew it could be withdrawn before the sale, and I don't think many people would bid under those circumstances. I don't think eBay would be happy if you made a habit of doing it.