I see a few potential issues here though, before we start getting to brass tacks.
For your system to work at all, your web server database has to be absolutely up to date or any new bookings could overlap existing bookings. This means it has to know when every customer bought his or her slot of ads, which space they occupy, when they run from and for how long since without this data nobody will know what is or is not available.
The only real way around this is to go into a small scale e-commerce operation, otherwise sooner or later you run the risk of someone tryng to buy a slot that has already been sold, only you haven't updated the online database to reflect that sale.
Whatever someone buys (advert position 10 for two months starting August, as an example) is charged to them, they pay for it, and your web server database updates on completion of the transaction to tell everyone else who may be interested in that slot that it has been sold and is no longer available.
It's not the sort of thing you cook up overnight and it isn't the kind of thing I can just say, "do x, y, z and all will be well".
The trouble with databases and data driven sites is that planning and designing your database is the key to it all working, or not, and going beyond simple queries on one or two tables of records to running appends based on purchases, creating tables or account holder records, details of processed orders (date, time, amount) and similar is where you can fall flat very quickly.
Have you considered setting up a manual system where the available dates and slots are posted by you as a web page ? Your customer selects from those options in a form and submits via email. On confirmation of availablity they pay you, and on completion of the transaction you update the list of availablity on the site, effectively removing the slot you have just processed from further selection by another customer.
It may sound cumbersome and it is when you compare it to a fully fledged e-commerce system. It relies on a lot of manual processing by you. Compared to learning database design along with programming the necessary scripts to effectively use your database, it suddenly may seem a more attractive option though.
To give you something coming close to useful help I would have to know the exact ins and outs of what it is you are selling, how you intend to process the orders (PayPal et al), will it be on a per month basis, will you offer discounts for long term bookings of 3 or 6 months (just as an example) and if so what percentage will the discount be ?
Even without all of this I can tell you that the likelihood of you being able to set your system up using Access without writing at least some maual VBA code is probably quite slim.
Data driven sites can turn into a serious nightmare and most of the problems originate in the underlying database design. I'd love to say it's dead easy and it will be a breeze, but it isn't unless you have a thorough understanding of two things: your intended selling process (what you sell, how long people buy an ad for, its position and so on) coupled with database design and implementation.