I'm in a social guild which sometimes has a trader and sometimes doesn't. I'm also in a social guild which never has a trader.

In the first guild I have continued to sell stuff during times when we didn't have a trader. Not quite as much as when we do, but items still go. I'm not one of these people who normally makes millions of gold on sales though - I usually just sell Intricate weapons and armour from crafting writ rewards, recipes I already have and occasionally materials (usually food) I have too many of - so it's hard for me to say how it would impact the people farming and selling high-end items.

In the second guild however I periodically list some items, which are the same types of things I sell in my 'trading guild' but almost never sell anything at all. I think because it's purely a social guild, the leader doesn't mention selling through the guild store and we never have a trader very few people ever list anything, so very few people ever look at what's being sold and the chances of someone who wants what you're offering seeing it are much lower.