In the Piazza Navonna we saw so many artists selling their wares. I dont think they were all the ACTUAL artists though, as some of them had varying styles within the paintings they were selling.
So, although it sounds great - buying an original painting from the original painter... I highly doubt this is the case. It is quite commercialised.
Do the rounds when you see some artists, note which ones you like, and then get costings. Tell them you can get a painting for less at Stand X, and the chap you are talking to will give you a lower price.
If you buy more than one, ask for a discount. They are readily given, albeit amidst some grumbling haha
Having said that, we bought 4 lovely works of art, 2 obviously Roman, but the other 2 not.
We have had them framed and they sit proudly in our sitting room (along with the other artwork we have collected from other places we have visited).

