As long as you confirm the trade on both ends it's really preference. I usually wait to be paid to send an item, and more often than not pay for an item before receiving it, usually there's little to no wait time as well so it doesn't really matter to me. Only thing I know people are more picky on is with art commissions and stuff, some people will require you to pre-pay or at least put down a deposit/partial payment so they don't end up doing work for nothing. But such will usually be stated in the thread.
I usually ask for payment first when handing over an item, villager, or collectible, but it really depends on the person's Wi-Fi/Trade rating. If it's high, I'm fine with delivering first. If it's low, I ask for payment first.
If collectible I usually ask for money first for a reason, however if I do in-game trades I prefer to give/receive bells after because that connection can break too easily.