I was recently asked to write my ideas, from an artists view, on purchasing art. Not sure how helpful this is, but I'll give it an honest opinion.
The first thing I see quite often, is that most buyers do not negotiate when purchasing art in person. Personally, I do not take offense at a lower offer. If I have room to come down on the price of a particular piece I will, especially for a returning client.
There are many reasons a particular piece of art is not ready for sell yet, or is priced higher: The photography may not be completed yet, or it is registered for an upcoming show, or the paint hasn't cured long enough to varnish it yet. Whatever the reason, don't be afraid to ask again at a later date, the artist may be ready to sell, and may even accept your offer.
Along the same lines, I love it when someone wants a custom painting similar to one that I'm not ready to sell. We can create it in a different size, change the color scheme, add personal touches, and create a new original that can be purchased with installments as we progress through the painting. This is great for those purchasing art for personal benefit, and not just as an investment.
If purchasing art on line it is important to remember that monitors vary, and that the colors usually look better in person. If looking to invest in art, it is best to do some homework. One place to do this is Invaluable, a fine art auction platform, which recently published a blog post of a Guide to Buying Fine Art. I recommend you research the artist, check to see how he or she compares to other artists, and whether they have reproductions available or not. The reproductions are not as valuable as the originals, but if prints are selling, then it is a good indicator that the originals are worth quite a bit more. Remember some artists are just getting started, and one day their work may be extremely valuable.
That's my professional opinion, please feel free to leave feedback, even if you disagree!