Transferring purchased software one ID to another in order to use just one apple ID

Not from your end. If you really want to, you can click here and ask the iTunes Store staff for assistance, but they probably won't do this.

(127660)

That cannot be done. Each Apple ID must purchase its own separate copy of apps either from the iTunes Store or the Mac App Store. Purchased apps are permanently tied to the Apple ID used when purchased.

You may want to pick one as your primary apple id. You can have one, two, or three apple ids per device. You do know that you can install the apps on five ( perhaps ten ) of your idevices from the same apple id. If you have two apple ids, you can log out of one. Your apps will stay on the device. If you need to update them, I believe you will need to login with the correct id.

Let's wait and have this reviewed for correctness.

Robert.

As mentioned, you cannot merge or transfer apps or Apple IDs; however, you can do this (excerpt from the App Store ToU):

APP STORE PRODUCT USAGE RULES

ℹ If you are an individual acting in your personal capacity, you may download and sync an App Store Product for personal, noncommercial use on any iOS Device you own or control.

(ii) If you are a commercial enterprise or educational institution, you may download and sync an App Store Product for use by either (a) a single individual on one or more iOS Devices used by that individual that you own or control or (b) multiple individuals, on a single shared iOS Device you own or control. For example, a single employee may use an App Store Product on both the employee's iPhone and iPad, or multiple students may serially use an App Store Product on a single iPad located at a resource center or library. For the sake of clarity, each iOS Device used serially by multiple users requires a separate license.

(iii) You shall be able to store App Store Products from up to five different Accounts at a time on a compatible iOS Device.

(iv) You shall be able to manually sync App Store Products from at least one iTunes-authorized device to iOS Devices that have manual sync mode, provided that the App Store Product is associated with an Account on the primary iTunes-authorized device, where the primary iTunes-authorized device is the one that was first synced with the iOS Device or the one that you subsequently designate as primary using the iTunes application.

You do need to use the Apple ID used to obtain the software. Here are the full terms:

http://www.apple.com/legal/internet-services/itunes/us/terms.html#SALE

What can I do with items purchased where the userid used for those purchases is no longer available for login ?
Can I transfer such purchases to my current id or need I accept that these apps are lost ?

You cannot transfer those - they remain tethered to the Apple ID used to obtain them.