We wouldn't say only you but most you together had really looked for your classifieds script's URL to change it's look to better shape. Not even this, some had asked for sub-domain according to state, city etc. And now we've listened to it and developed such module which does not require CPU power anymore unlike last time. It is very simple technology that we recently developed that which will let you have your own http://city.yourclassifieds.com easily without need to change for lot's of stuffs.
This feature is very much useful and even let's your users remind their city based URLs. This is more like craigslist.org website as they're using the similar URLs for their website, its the same reverse technic done with our classifieds script. Currently this is an optional feature for our classifieds script since not compatible with all types of site as explained in next paragraph so it is available for sale online under our AddOns section.
This also helps your site to index on google easily and helps increase your PR (page rank) of your main domain easily while having holding so many sub-domains.
This modification will not only change the city(regions) URLs, but it will change the whole format of the URL for other pages as well, such as viewing categories, viewing ads etc. The experience becomes different than earlier and much for friendlier than the default addresses. Why don't you check out our demo as given on our demo website located at http://www.phpclassified.com.
We're still on the development process of such URLs at this moment but while we are working on it further, we're able to release a simpler, easier version of it which works pretty well but it has a minimum requirements. Only requirement this modification need is that your classifieds website is running directly under root directory of the domain. Let's say if your website is installed under www.your-classifieds.com then "yes", your site is compatible. But if your site is installed under sub-directory for example www.your-site.com/classifieds/ then most likely you may not be able to use this function. Since it works directly with your domain to use it's sub-domain to send users to the appropriate city so root folder is necessary. Finally but not least, your hosting company must allow wildcard subdomain additional such as *.yourdomain.com must be added under your sub-domains list by you. Which is allowed by most hosting providers so should not be an issue unless you're on a free hosting which is unlikely too.