With the limited nature of many lolita items, and the number of people in the community who enjoy the same things, it’s inevitable that eventually you and someone else will want the same item.
If you happen upon someone else’s want to buy post, and they have someone offering to sell them the item, it is considered to be quite rude to message the seller or comment asking the seller to sell to you instead.
If you feel you must take some action, it is better to make your own want to buy post, and see if the seller offers you the item (or another seller offers it).
Likewise, if someone posts a wishlist and they are contacted by someone because that person is selling the item, or a link to the item being sold somewhere is posted as a comment on it, it’s in poor taste to swoop in and try to buy the item.