Hello Sir,
[login to view URL] a "sticky" buy now button
=> You need sticky button on product info page?
2. on account login they can see previous purchase and reorder same item without going through whole catalog
=> You want add last products order and its redirect on cart page?
Also I suggest update you zencart version, its too OLD its not work PHP 5 and MYSQLI
if you want I will patch your zencart with all upgrades files which around 8-9, its take 6-7 days and cost around $300
Regards,
saiSoftIndia