I am of a different opinion on this topic.
If someone is just running a generic "I want.." ad for things that we all want (Red Indian Tins, Rainbow globes, Buffalo signs, Case Eagles, Dealership porcelain neons etc etc...), I see nothing wrong with others piling on and saying they want one as well. That is better that seeing a dozen ads of people looking for the same thing.
If however someone drafts an ad for a specific need (ex. a specific globe face to complete a globe, a specific pump part to complete a restoration, a specific sign to complete a set etc etc...)I agree that others hopping on to that ad may be considered rude.
Just my 2 cents.