It depends on the nature of the work and what you are requesting.
When I was a new seller, I’d send free short demos. After someone has a few hundred sales, there are plenty of examples of their work (or there should be).
Part of it is what are you requesting and how much.
Prices on Fiverr are so low, experienced sellers rarely send free samples, when they have a large existing body of examples.
If you have an important project with an experienced seller, consider buying a small $5 to $20 sample, to see how the seller handles it.
Sellers are often asked for free samples from people who just use it if they can. (Depends on what you are really asking for…)
On the other hand, I rarely do free samples anymore. If my 5,000 sales and samples I have available on Fiverr and Soundcloud aren’t enough to show what I can do, then we are not likely to work together.
It really depends on the nature of your demo request, and the level of the sellers you are selecting. With prices so low, doing free demos isn’t realistic.
BTW - This probably doesn’t apply to you: Many buyers promise “lots of future business” or “a large gig” and then don’t deliver on either of those. Experienced sellers get to the point where they don’t expect or believe someone will become a repeat buyer until they have a few month track record. Too many people try to get free work, and it hurts someone like yourself who is legit, and just wants to kick the tires and then decide.