Just to give you an idea of what we deal with -- both coming in and going out - and some of the things we correspond about.
Most of my correspondence starts as eBay messaging. eBay blocks messages containing url's or email addresses. They also will not allow them in sales listings. I must encode web addresses in mine. If an exchange runs more than a simple question and answer, I urge my correspondent to switch to regular email - where I can at least send them page addresses. Most people are too obtuse (thick-headed) to comprehend this. No matter that I say: contact me at support at bigtwins dot org in every exchange, they rarely do.
I don't know how closely eBay looks at every msg, but I'm a touch paranoid. From each transaction they take from eight to twelve percent off the top -- including shipping fees. Thus everything onsite is cheaper than the same parts offered on eBay. There is still little comprehension that it costs the seller something to sell there.