#1: Quantity.With any paving job the general rule of thumb is to allow an extra 5% for accidents and cuts. Second hand pavers have probably had a bit of wear and tear and/or stains, and may suffer even further in the process of lifting and transport. It would be wise to allow an additional 5% for this (total 10% extra). Also, don't count already cut pavers - if the seller doesn't identify how many whole pavers they have, ask. If they can't (or don't) answer, allow another 5% to your minimum requirement (total 15% extra).
Remember: Unless the seller can identify exactly where the pavers were purchased from, and new pavers are currently available, then you can not get more if you run short.
Example: Your area for paving is 40sqm. Your minimum requirement should be: 40sqm + 2sqm (cuts, accidents) + 2sqm (stains, lifting, transport) + 2sqm (previously cut wasted pavers) = 46sqm.
#2: Colour.Rarely does a photograph indicate the true colour of a paver. Where practical, ask the seller if you can collect a sample to take home and try out in its potential future position.
Pavers are often made with a bevel around the top edge. The greater the bevel, the more rubbish rests in between the joins. To make sweeping easier, pavers can be laid upside down. The added advantage of doing this with second hand pavers (particularly with concrete pavers which fade over time) is that the underside could look almost new. However, sometimes pavers are laid on a mix of sand and cement - this stains the underside of the paver. Cement stains can be cleaned with an acid wash, although it may not be worth the trouble.
The moral of the story - check out the underside for potential use as the visible side, and take that into consideration when you determine your bid price.
#3: Transport.When pavers are bought new, the supplier will typically arrange for delivery to your location for a reasonable fee (sometimes for free on large quantities). The pavers arrive stacked on pallets which are very heavy, large trucks and forklifts are required, and often (depending on access) the pavers can be placed quite close to their potential final laying place. You should not underestimate the value of this delivery service in minimising unnecessary handling of pavers, especially with larger quantities. If you are required to do all the lifting, stacking, handling, transport, unpacking, etc, there is a significant physical toll you will need to pay before you even start the job.
Having said this, I strongly recommend you consider the complete cost of transport in your budget, as well as the physical cost in the extra work. Remember: pavers are heavy. With smaller quantities, a couple of trailer loads may not be too taxing, but for larger quantities (e.g. >25sqm) whack them on pallets and get the big boys and their forklifts and trucks to move them - your back will thank you for it.
For more general info about paver types and laying techniques, here's a website by some very knowledgable pommies (if these guys don't know it, it's not worth knowing):