I am afraid this will be very hard to do.
First of all, most of the artistic oil paintings are very individually estimated, and one can purchase a painting for a lot of money while the other wouldn't bother to look at it.
Galleries are estimating the paintings displayed assuming that the person who's looking at them likes the painting a lot, and also by how much work and time the artist seems to invest in the painting. although, you can understand it's never an exact science.
Therefore, I'd doubt the possibility to have such a service online. although, I'm sure you'll find some art valuation service in your local yellow pages.