classified site, with a little difference

I want to create a site, where maximum amount you can sell / place advert for is (R100 - our currency uses (R) symbol ) / $100 if it makes more sense using $ symbol... for example, the entire site concept is to sell something in classified under 100 only.. you can't sell something for 150... you have to sell it for under 100., but I need to create a customized landing page, which i can create, but need your help - creating the classified part of site and make it so, no one can put in over 100.. if you put 110, advert will be taken down.. Basically it will be a site (classified site) where you can post stuff for sale and jobs under 100.. eg: For sale: I want to sell playstation: R99,00 post it on my site.. Jobs: I can create you a website for R100,00 post it on my site.. The currency we use in my country in RANDS.. Symbol (R) eg: R100,00 Website must use this currency. Website will have a customized landing page, created by myself... I need you to create classified part.. post advert.. The website will be filled with adsense and advertising spots, please make space for adverts..