I agree with others in general who have written that you should only buy coins that have really clear, sharp pictures showing lots of detail.  But if you have bought coins from a seller before and the coins have been consistently good, sometimes that trust can pay off  when a picture is bad.

For example, I paid $125+ for an MS 1854 Quarter recently.  Judging by the seller's picture, the coin appeared to have a large area of pitting (a large depression) on the right side of the obverse:

The Reverse also appeared to have a planchet flaw near the letter F in the word "OF" in the original seller's picture (may be hard to see in this smaller picture):

But I had bought many coins from this seller before and they have always been consistent in quality.  So for $125+ I was willing to take a chance on this 1854 Quarter.  The seller did not indicate there were any problems or damage on the coin either.  The seller also had a solid return policy.

I've taken a better picture below, and this is the obverse of the same coin:

And here is one of the reverse that I took:

Imagine my surprise when the areas I thought were planchet defects, pitting, or whatever turned out to be areas of toning.

I also thought this was a wonderful example of how pictures of coins can be very misleading.  But in this case it worked out well because I trusted the seller.

I do not recommend buying coins from sellers you do not know if:

1. The pictures are blurry.
2. The pictures are too small.
3. The pictures appear to light and washed out.
4. The pictures appear too dark.
5. The picture shows possible problems like damage, lots of hairlines, etc.

For numbers 3 and 4, remember contrast and brightness can be adjusted to make a bad coin look better.

But if you trust the seller and the seller has a solid return policy, you might want to take a chance.