Does 20% deposit consider a good consideration?

Let say i saw an advertisement on the shop outside, and there is no pice list.

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

is the 20% deposit count as a good consideration? and is there a contract between me and the shop?