The usual reason for washing before sewing is to make sure that the fabric has done all the shrinking it's likely to do. This fabric is polyester, which doesn't normally shrink, and one doesn't ordinarily wash upholstered furniture.
But I also think you made a poor choice of fabric, though you didn't say if you're re-upholstering an armchair or merely stapling some fabric over dining room chair seats. That fabric would be acceptable, though not great, for the second use. For the first, I think it will be very hard to work with and probably not satisfactory in use.
The ad does not say it's intended for upholstery. You were looking under the "Type" category when you saw that, weren't you? That's the ebay category. Scroll further down where it gives more info about the fabric. Says it's suitable for dance wear, evening wear, skating costumes. That's because it's thin, stretchy, and drapey. Not what you need in an upholstery fabric. Too bad the seller didn't give care instructions for the fabric; most panne velvet can be machine washed in cool but not machine dried.
Next time, as a novice, go to a fabric store where the sales people can give you advice about what fabric to use for what you're doing with it. I've been buying fabric for over 20 years and I'm still not sure I'd buy fabric online because I'm just not familiar enough with all the terms used to describe it.