No, you cannot buy the 24 points on-line (and note that even by phone you can only buy those 24 points when making a reservation 7 months or less out). The system allows you make a reservation using banked, current or borrowed points. You need to bank the points before using the reservation section of the site but you can bank your points on-line in another section. The reservation section allows you to first pick resort and room category and your dates. You then go to a page that tells you whether it is available. That page also gives alternatives at either other resorts (only if booking at 7 months out unless you also own at another resort) or different room categories in your own resort. If what you want is not available, that page also says you can waitlist and there is a icon to to take you to the waitlist page.
If what you want is available, you click the icon to make the reservation and then you go to a page where you choose applicable contracts to use. As soon as you get to that page, the reservation you want is reserved for twenty minutes to allow you to complete all things you need to complete. Once you allocate points, including borrowing points if needed, you click to the next page where you will fill in guest information (names, addresses, and phones to the extent you have them). You are not required to fill in any more than the lead guest (meaning you usually) but you cannot latter modify this information on-line; instead, you must call to add anything. After you have confirmed points and given guest information, you go to a page summarizing what you have and you make the reservation. You then get a confirmation page where, if you want, you can still cancel the reservation before leaving the page and if you do all points go back to where they were, including borrowed points go back to original year. In other words, if you blew it, you are given one last opportunity to wipe out your mistake. After you complete the reservation they send you an email that provides a link where you can go and see your confirmed reservation.
As noted, you cannot buy the 24 points on-line. You also cannot modify your reservation on-line, such as adding a night, and have to call to do that. The only way to change a reservation on line is to cancel the one you have and start a new one.
Bill 
