Buying speaker cables on-line: Is this advisable?

Optimal cable selection really requires comparisons of various brands in your own system. There is no other way to make an optimal selection.  I don't think the information a manufacturer or dealer can provide to you will reveal how the speaker cable will work in your system.  Of course, you can buy a reputable speaker cable without ever having heard it, and it might be adequate, but you will never know if it is optimal or merely adequate.

Assuming you want to make comparisons, then there are two approaches you can take.  You can "borrow" cables from manufacturers who offer money-back guarantees or in some cases from dealers.  The benefit of this approach is that it involves the least hassle. If you don't like how a speaker cable sounds, you just return it and get your money back. The downside is that you will end up paying full retail or close to it for a speaker cable, unless it happens to be available on the used market.

Alternatively, you can buy used, try the cable in your system, and then resell it if it doesn't float your boat. The obvious benefit is that you get a discount in the used market. The downside is that it can be a real hassle to resell cables and there is some risk that you won't succeed in reselling them without accepting a substantially lower price than you paid.

I personally don't like this approach, particularly because speaker cables are more difficult to sell on the used market than interconnects and power cords.  I almost always have bought cables new from dealers or directly from manufacturers after having heard them in my system.

Cables are the same as anything else. Only difference is buying used the selection is a little different. In my case I know from decades experience that Synergistic Research makes excellent cables. So I look for used in my price range. Then when I find one the next step is to search for reviews and listener comments. Construction and technical comments are frankly irrelevant. All I care about is how it sounds. Read reviews and all the listener comments you can find. Search around the forums for comments.  

The last one for example was Synergistic Element Copper Tungsten Silver, which was a $7500 speaker cable when new 15 years ago, only about a grand by the time I bought it. Awful lot of cable for a grand. Just to give one example. Synergistic is highly recommended when looking used because they have been making quality wire for so many years, there's so much of it out there, and its very reliable in terms of sound quality. If you like a thinner faster more hyped sound then you will gravitate elsewhere. Point is its all based on what you want in terms of sound quality, and what others say they hear in terms of sound quality. You match what they say they hear with what you want and when the two meet together at a price you can handle then you go for it. That simple.

There are many choices for speaker cables out there. If you have an unlimited budget, I think you should buy every available cable, then select the one you like the best. Failing at that, I think you should follow millercarbon’s advice and read reviews from members here on Audiogon. At some point, you should have enough confidence to purchase a set of cables. I recommend buying a used pair, by a reputable manufacturer. There are no "BEST" speaker cables at any price. If you find the pair you like, keep them. If not, keep looking! There are no magic bullets in this hobby, it’s mostly trial-and-error. That’s why we all have so much of this stuff.

Only buy if you can return. There's a website called The Cable Company where you can do that.
https://www.thecableco.com/lending-library
The variability is too extreme to rely on opinions. 

The Cable Company has a sister sight called Usedcables.com as well. For the most part reasonably priced. TMR (The Music Room) sells regularly on Agon and has a generous return policy. Some other dealers sell demo gear, usually can be returned as well. Just have to look around. 
To another post, I have a set of Blue Jeans cables. For the $$$ not a bad cable... but they simply don’t compare to the likes of Shunyata, Analysis Plus, or Transparent. I don’t know other brands first hand, as mentioned the best thing to do is try some... they all have pluses and minuses. You just need to figure out which suits your system and your ears.