Have chance to buy used (2010) blue pearl 325.00 Seller said it was only used about 12 times i know there is no way to confirm that. I have 20X40 vinyl with steps is this a good vacum for vinyl(no damage) is it too big of a risk buying used

If it looks to be in reasonable shape and it works, I think that is a good price. The lifetime of pool cleaners is typically around 3-5 years. That is a good enough discount off the new price that it seems worth the risk that it is actually used far more than they describe. Also, if it was actually used heavily it will show a fair bit of wear, which would be reasonably obvious. The only significant risk is that it doesn't work at all. If you can cover that by either testing it before purchase or getting an agreement to get your money back if it doesn't work at all, then you should be fine.

can you test them out of water?

I don't think they will do anything when out of the water.