Doesnt bother me much if they dont post a price, I will always ask a trader the best price they can do for me. If I've been a loyal customer, I'd expect a bit of extra discount. When I was hammering my 350z up and down the motorway and getting it serviced every 5 months, and getting it serviced at the same place every time, I got more discount, and rightly so, I was giving them good repeat business. I dont see this as any different.
However, I do prefer if a trader posts up an item specifically for sale then they give their current price for the item