Marketing in small towns is going to be difficult. A lot of small towns lags behind more urban places in internet adoption. Their population are also generally older and less tech savvy than cities - so you should plan for this group (simple interfaces, etc...).
You also get small towns that are relatively near each other: if a person wants to buy something $200 + he will happily drive through to the next town. So if you implement a type of map and list nearby towns it may be good.
A lot of selling over here happens with small regional papers – a regional paper will cover one or two towns and will be filled with regional news (stuff like school performance, local politics, etc...). Those papers also contain classified advertisements. If you can maybe link up with one or two of them to put those advertisements online – or provide a system in which they can put advertisements on your system (e.g. show on their website and redirect to yours when a user clicks on the add) it would be great.
Most of those papers are usually a one or two man operation without much of a website – you should check if you can get a system in which you supply a CMS type system with your advertisement system to them. You provide the website framework and advertisement system and they fill it with content & advertisements.