You don't really need a separate account for the discussion boards, but it certainly pays to have separate buying and selling accounts. Buyers have been known to go through feedback to see what you've paid for an item, and then get all uppity because you have the audacity to ask more for it than what you paid. It can also send buyers to that seller to get it at the same price you paid.
I have 3 active accounts. I haven't bought anything for ages, but when I was buying, I'd buy on all accounts, but wouldn't list the items on the account that bought them. I would label every purchase with the amount I paid and the account I bought it on so I wouldn't forget. Prior to doing that, I did have one buyer abuse me because they could see I'd paid $10 for it and listed it for $15 + postage. She got blocked almost immediately.
