Depends whether it is a "shop" or an individual.
When you say if its not suitable, do you mean if you change your mind, the wrong size or just don't like it? Then I would say, No, unless the seller says you can return it (at your own cost) for a replacement/refund.
I bought a spare part for an electrical item, it was supposed to be a "one size fits all" but it didn't fit. I was allowed to send it back and got a refund, but had to pay the postage both ways.
Say you bought a dress from Freda Bloggs and when it arrived you didn't like it, I would say the answer would be a definite No.
I always state in my listings, "No returns, sold as seen in photo".
That is pretty clear.