There are a number of websites offering free adverts. But the most important thing is if the y got the traffic that will convert into customers.
For example, mzoori, businesspalace, mocality or dealfish.
But if you want to go professional its important to own your own website that will give you the freedom to do anything you want with your business. it will above all give you the advantage of posting as many adverts as posssible without limits.
Take for example, this websites
www.dresscode.co.ke that sells clothings and
www.tembeanami.com thats sells shoes, they are marketing tools but personla making it possible for the owner to make changes without limitations and with full control of his business.
It will cost you about 20k to design such a website but more effective in the longrun and more profit raking.
Dont hestate to inquire for more info if you need further infomation