As an eBay seller, you've no doubt discovered that eBay wants a lot of data from you:
A working phone number
A real mailing address
A real email address
Bank account information
Sure, it's possible to wriggle your way to an account lacking in these things over time, but if eBay finds out, they'll suspend you.
More upsetting for some are parts of the eBay user agreement and eBay policies that imply that they'll contact you anwhere and anywhen they want, and may even make records of these conversations.
What's a privacy advocate to do?
And are eBay planning to resell your data, or to use it to launch a marketing business of their own, in which they pepper you day and night with ads from third parties?
The answer is no.
eBay isn't trying to snoop, nor are they planning to resell your personal information or use it as a way to call you at dinner time in order to sell you diet drugs or a political candidate.
Long Answer: The Reasons
There are a variety of reasons why eBay has implemented these policies and parts of the user agreement, but they all come down to one thing: making it possible for eBay to do business.
Just about anyone that has bought or sold on eBay for any length of time becomes aware of the all-too-conventional wisdom that eBay doesn't care about fraud or that eBay is full of scams against buyers and sellers.
Of course, it can't be that eBay doesn't care about fraud or rules and that they suspend people right and left all of the time due to their overzealous policing.
In fact, here's what eBay wants your personal data for, in order of importance.
To ensure that you're serious and accountable. The most basic reason for eBay's requirement for lots of real contact information is that traders are much more likely to behave themselves (particularly in relation to illegal activity or fraud) if they know that eBay knows who they are and where they live. Most users receive neither mail nor phone calls from eBay.
To process the payment of seller fees. eBay sellers list items and incur fees that they're only asked to pay once every month. Having personal and bank account information online allows eBay to (a) actually process selling fees payments from your account if you're a seller, and (b) contact you about large amounts of overdue fees or take appropriate collections actions if you're ever in a position to owe large amounts of overdue fees.
For law enforcement purposes. Of course, when something illegal does happen on eBay's site, they provide personal information to law enforcement agencies that have an interest in the case. This, too, is about keeping dishonest buying and selling activity to a minimum, as well as combatting organized crime, which is always looking for a way to sell stolen or counterfeit goods to the public.
So that other eBay members can contact you. Keeping in mind that eBay wants members to contact one another in order to resolve disputes, it's untenable for eBay to allow members to trade on the site with false or missing contact information.
So that your accounts can be cross-referenced. If you have multiple accounts, eBay wants to know. This keeps bad traders that have ruined their feedback or committed violations like shill bidding from simply registering a new account each time they are caught damaging others. By making sure that you provide details like your name, address, phone number, email address, and bank account information, and by tracking additional data about you that can be determined as you access the site, eBay is able to link newly registered accounts to existing accounts owned by repeat violators and disable them immediately.
So that they can contact you. Yes, as a last resort, there may be a circumstance in which eBay needs to contact you, but in practice this doesn't happen often. Calling or mailing you costs eBay time and money that they really can't justify spending on most traders; they're much more likely to simply suspend an account or refer your contact information to law enforcement if it is requested as a part of an investigation. In practice, most traders have never heard from eBay without contacting eBay first, and most never will.
For some prospective or existing eBay users, these reasons make good sense. For others, the thought of giving eBay access to this information may be too much to bear. The choice is yoursbut remember that not having real, working contact information and personal details on file with eBay is likely to mean an end to your eBay trading days, sooner or later (and probably sooner).