Loan parts with no repayments

Three or four weeks-worth of interest, zero risk of default.

As a percentage, the numbers work the same either way, and £20 parts sell much more easily than larger parts. The only question is whether the accrued interest minus the selling fee is worthwhile.

For a £20 8% part, you earn about 13p/mo, minus 5p fee. Not, IMHO, worth it, especially since the risk of failure before the first payment should be very low.
For a £20 18.2% E-band, minus 5p, with a higher risk, it's much more worthwhile...

The first payment's usually fairly safe across all bands, not least because FC have treated failure there as fraud before and repayed in full - but there's various schools of thought about the second-to-sixth payments being the optimum churn time, depending on band.

Obviously, the correct answer is to sell just before the first payment that's not made...

There is always the risk of the loan being downgraded before the first repayment because of a 'credit event', for example a CCJ materialises to the great surprise of FC and the Borrower. In that case you can be locked into a loan which may pay for a while and then go bad. This sort of thing seems to be happening with some E loans - though I have not actually lost anything yet. Flipping before the first payment is not completely safe.

I've collected reams of statistics over several years and I still can't reliably make it happen (although I can demonstrate that the penalty for getting out too soon is significantly less than the penalty for not getting out until too late).