Plenty for sale in SM ?

Hover your mouse over the amount available in the loan lists, and a tool-tip appears showing the best discount. Try it on the Leeds loan which generally is trading at a discount.

Then open up the loan page, and click on the link for the amount available. A popup will appear showing the amount available at each discount level.

EDIT: cross-posted

... and which haven't already been bought.

Just as with purchases at par, it isn't necessary to wait until units are actually offered for sale before submitting an instruction to buy at a discount.

For many of the loans, even with significant quantities available at par, there's unfulfilled demand at a discount, so any units offered at a discount will disappear "instantly" (which can admittedly be quite slow due to various bugs and inefficiencies in AC's systems, but in any case they'll not last more than a couple of hours, with the majority gone in a few minutes if the demand is there).

The main user-visible effect is that all but a small amount of the loan units made available for sale get sold... and then the next time it cycles round to that loan, it sells all but a tiny amount of that, and the next time all but a misicule amount of what was left the first two times, etc. until the remainder is smaller than AC's rounding factors, and so gets ignored by the system.

In particular it does not seem uncommon to see a tiny amount of loan units left over from an earlier sale (and in particular seeing a loan showing availability with a 1% discount, but clicking through to find that the amount at the 1% discount is far smaller than a penny).

One cause of this effect seems to be that at least one part of the system applies the 1% discount to the wrong number, so that if I've got (say) £100 in my account, and am buying a 1% discounted loan, I end up buying £99.00 of it for a price of £98.01 (of course, the actual numbers are never this round!). It was discussed some time ago, and I'm not aware of that specific one being fixed, but I'm sure there are other causes. Not knowing the detailed implementation, and having no way to trace the cause, I just roll my eyes when I see ridiculously small transactions clearly caused by some kind of rounding bug rather than trying to figure out a reproducible test case.

To my mind, it's one advantage of (at least internally) dealing with purchases and sales of fixed-price "shares" (with no more than 10¹⁸ of them per loan, and all accounting exact to 20 decimals, as discussed on the other thread), rather than of "a GBP amount of a loan that's internally represented by a % of the loan to so many decimal places that we can ignore any rounding errors" - it gives the programmers more clarity when tracing through such bugs (or when implementing the original design), and ensures that test suites can verify the EXACT accounting rather than needing to make allowances for rounding errors.

I also personally feel it "incorrect" that if I offer to sell £100 of my loan unit with a 1% discount, it actually sells £101.010101... of my loan units in order to raise £100 of cash. It also feels a bit weird from the buying side too - I've never been to a shop where a % discount is applied and find that they give me more of the item than I asked to purchase rather than charging me a smaller amount.

I'll look into the other bits over the next few days.  Wasn't aware of them but the rest of the team may have been.

With the bit quoted above I actually agreed with you to begin with, however I have been reliably informed that the implemented method is how share trading platforms work so it's the way our buy / sell orders have been designed to work.  It does make sense from the other point of view that "I'm looking to invest £100 and will accept anything at par or better", then discounts get you a larger holding.  You don't know what offers you're going to get, so if you wanted to invest £100 but someone offered a 10% discount, you'd be annoyed if the system didn't invest your £100 but still declared the buy order as having been complete.

So now I agree that the current system is correct, although I do see where you're coming from.

Not sure which share dealing platform(s) you were basing the design on, but the one I use gives a choice between "shares" and "amount in currency", with "shares" pre-selected if I don't choose otherwise, and the very limited number of other share dealing platforms I used years earlier always had me input the number of shares and not an amount in currency. When buying, the "amount in currency" can make a lot of sense for the reasons you highlight. When selling, I'd think it far more likely that the number of loan units would be the intended amount (if I've £200 in a loan, and ask to sell £100 at a 1% discount, I'd expect to receive £99 and have £100 of loan units left over. Ending up with £98.989898... in that situation seems surprising to me - one could try to fix it by buying £1.01). However, I can still see the potential value in being able to specify the amount of money you wish to receive rather than the amount of loan units you want to sell.

AC ends up with a confusion in terminology because both the investment amount and the quantity of loan units are denominated in pounds and pence (and fractions of femtopence!), and the UI doesn't make it clear WHICH is being entered (indeed, to me it seems to point to the quantity of loan units rather than the cash amount).

As for your example... I'd be glad that the system had done exactly as I'd requested, providing me with the £100 of loan units I'd asked for. But I'm coming from the position of having buying orders far exceeding the total of my available cash, and I can certainly see where someone who'd get annoyed at this was coming from (e.g. added £1000 and put £100 buy orders on exactly 10 loans expecting this to use all their money).

Or perhaps a nice juicy cash back offer.

There are broadly similar loans on TC, with plenty of choice and rates typically 10 - 13%. If TC can shift them at this rate why can't AC?