does 20% deposit consider a good consideration?

Let say i saw a advertisement on the shop outside, no pice list.

I would like to buy a pair of glass in the advertisement from shop, and the owner was told the glass is worth 1000, and the shop dose now have it right now, and the shop ask for 20% as a deposit and he said the glass will be available next week.

is the 20% deposit count as a good consideration?