I need help with the configuration of my magento

:( My magento site that is hosted by Linode was setup by a guy who didn't quite know what he was doing as far as system administrator.

Various random online clients cannot checkout, however when I login as them..I'm able to checkout for them…What's up with this. One programmer said that they should update their browsers…but I can't keep telling them to do this. Keep in mind…these are customers who call from all across the country who are experiencing this. Yet, as I said I'm able to checkout for them on my computer. Linode guy said it was a system administrator issue and that it was not Linode.

Plus we need more work done on the magento platform like "one step checkout" / flyer applications for our customers to download etc….

HELP HELP HELP !!!!!!!

KAT