It would be the norm for the purchaser to indicate to you why they are not paying the deposit. I had a case before Xmas where contracts came back without a deposit and after weeks of very frustrating unanswered calls, faxes, etc.,etc it became quite clear that the purchaser was sort of interested but not enough to really try to want to clinch the deal.
I think if you sign the contracts without agreement on deposit ( i.e. that the 2% booking is intended to be the contract deposit) that it will be fine if deal closes quickly. However, if problems arise and deal falls apart, you won't have the comfort of the 10% deposit.
mf