From eBay (FWIW):
Policy overview
We don't allow our members to use eBay to contact each other to make offers to buy or sell items outside of eBay. Also, members can't use information obtained from eBay or any eBay systems to contact each other about buying or selling outside of eBay.
Sellers are liable for fees arising out of all sales made using some or all eBay Services, even if sales terms are finalized or payment is made outside of eBay.
Sellers are also liable for final value fees if they offer or reference their contact information, or ask for a buyer’s contact information, in the context of buying or selling outside of eBay, even if the item does not sell.
If you receive an offer to buy or sell outside eBay, please
report it.
Make sure you follow these guidelines. If you don't, you may be subject to a range of actions, including limits of your buying and selling privileges, restrictions on listings and account features, suspension of your account, application of fees, and recovery of our expenses in policy monitoring and enforcement.