Contacting an eBay bidder is a relatively straightforward process.
However, there are a few different situations in which you would be attempting to contact them, and the situation determines how you go about doing it.
Contacting a Winning Bidder
If the bidder you are attempting to contact has already won the item then the process is very simple.
Firstly, log into your account and head to the Seller's Hub section of your account.
From there you need to find the purchased item and use the dropdown menu where you can select “Contact Buyer”, and from there you can send a message to the bidder.
If it is a particularly urgent communication then you can choose the “View Sales Record” option, where you should be able to find the buyer's phone number.
Contacting an Active Bidder
If the bidder you wish to contact is currently bidding on an auction that has not finished yet, you can send them a message via one of two ways.
Firstly, if you know the bidder's username then you can simply go to advanced search, choose to search “By Bidder” and enter their name.
Once you have found the right user, head over to their profile and click on “Contact Member”.
Alternatively, you can go to the item page on which they are bidding, and there should be text displaying the current number of bids that have been placed on that item.
Click that link and, in the resulting list, find the user you wish to contact and click “Contact User”
Contacting Bidders You Have Contacted Before
While the previous two methods also work, I personally find that if you've already been in contact with a bidder you should continue the communication in the same thread.
The easiest way to do this is to head to the messages section of your eBay account.
Once you've found your previous communications with the bidder, you continue from there.
This allows you and the bidder to see the previous communication for reference should it be required.
What Should I Not Say to a Bidder/Buyer?
eBay has a number of rules about member-to-member communication, particularly as it pertains to a specific transaction.
It is best to avoid breaking these rules, as you may be reported and/or eBay may decide to take action against your account if you are caught doing so.
Offensive Messages, Threatening Behaviour, Spam.
Similarly, any offensive imagery (including nudity), threats against other members, and spam are all against eBay policy.
Attempting to Contact Bidder Outside of eBay
It is also against the rules to exchange email addresses, phone numbers, addresses, or any other kind of information that may make it possible to complete the transaction outside of eBay.
Changing the Terms of a Sale
Any attempts to alter the terms of a sale—or potential sale—will likely be reported to eBay.
If you have any conditions that are not displayed in your listing and you try to enforce those conditions after the sale is complete, particularly if it means asking the bidder for more money, eBay will almost certainly side with the bidder.
The same applies to any eBay imposed terms, such as mandatory returns.
If eBay requires that you offer a returns window with a particular listing, you cannot then message the bidder to say you do not accept returns.