Having it ask if you want to buy something every time is stupid, because then if it's a cheap item, and there is a group of people around, you all bid and it's a race to see who can get through the stupid "want to buy it?" menu first.
The time idea is much better and easier to implement.. just have it non buyable for 3-5 seconds when first put up.
The idea about saying "buy item 2 for 50" or whatever, that's okay, but it's more trouble to have to say all that, then just say what you'd normally say, and have to wait 3 seconds.