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When you are going to hire offshore Wordpress development services, find out their working expertise. like does they have diversity in their work, how creative they are by taking Wordpress plug-in development, portfolio, clients references and testimonials are good enough to judge a development firm.

martinluther is right, do not look only for cheap development. Quality is worth much more these days.

Developer should require from your:
- compatibility(do you want your plugin to work with older versions?)
- PHP version you need, there compatibility issues between versions
- very detailed functionality of plugin - better he as about more details. Remember coders are not mediums, they can not fit "Make it nice", because definition for nice is different for everyone
- UI description or image

if you just start and don't know people, some work first - money latter will be best. Freelancer sites is good to start - they allow "deposits" from you to be hold before work is delivered.

Please be smart - ANY details you elaborate after start, increase delivery time.

Somethings you can do is enable wp debug (simply changing a line to "true" in the wp-config.php) and make sure there are no errors. Also look for notices of deprecated functions, etc - it's important to use up-to-date WordPress codex compliant routines.

You could possibly request to review some of their most recent plugins they have developed and request that they enable wp debug.

In terms of deliver-ability request they do a trial run with you on a service such as join.me (or Skype screen-share). I normally do this when submitting work to clients.

What CrazyCoder said is not as important as their understanding of the WordPress codex because the core requires an updated PHP version - basically what he said should be "a given"

I create wordpress plugins, so if you ever need a contact...feel free to PM me