It goes both ways.. I've been shafted more times than I care to count when it comes to getting paid once the work is done. In fact, no one here on this site has paid up as agreed when I have accepted work from them and completed it.
They were my best bud until the work was done and then I could not even get them to respond to any correspondence. So needless to say, I tend to shy away from anyone on this site asking for help or for any work to be done.
For some reason, most just don't realize the amount of time and effort that goes into creating something like this.. http://billstelling.com/forum.php
That's also true.
If somebody accepted my deal to do a custom job, I will refer them to other vbulletin coders who I have done business with.
The more times you do business, the more trusted of a buyer/seller you become.
If they've never done business with anybody yet, then perhaps take a look at their post count/ see how much contribution they've done/how active they are in the community.
Before allowing anybody else to start doing the work I request, I first negotiate a minimum price. Whether or not I pay extra is up to me. However if I ask for extra suggestions mid way, then it's only fair that I pay more especially if they ask. If I refuse, they do not need to do any extra suggestions.
In any case, if somebody requests to do a job and you do it, but they no longer respond. You should probably sell that product on your website. Link to it in your signature, that you're selling mods on your website.
As the product developer, you should never release the product/give the buyer the product before receiving some (if not all) of the payment.