i don't know about it in the domain sales space but this is what works for me in the website dev space so it could help.
basically a payment plan is whatever you can mutually agree with ! to easy.
i've done 12 and 24 month plans on very large jobs and its worked out well, with dev however i've had to do all the work and pray they pay and luckily i've had no dramas, if they didn't pay each month the deal was the site would go down...... end of story till payment was made.
99% of the time i do a 3 month plan , 3rd to start , 3rd on design approval, 3rd on completion, i may complete in 2 weeks depending on client giving me the info or it may drag out but the last 3rd is generally on me completing not the client completing.
its important to put a DATE on the end payment, you need a "line in the sand"
other times i use 3rd 3rd 3rd regardless of time taken to develop.
which basically brings me back to "basically a payment plan is whatever you can mutually agree with ! to easy."
its a balance between getting your money and making or losing the sale.
also be prepared to walk away, i made a small sale this week and i don't sell many and the person was hard yards, in the end it worked out but i was prepared to walk away, it was only $xxx and i think if it was $xxxx + my attitude would be different.
i think the difference for a domain is there is no "work" involved compared to a web dev, + they can be given access to use it instantly without being given the rights to it until final payment.
1 domain i sold i gave access to use without any deposit and the deal was payment in 3 months, looking back that was stupid of me ! i should have at least got some money upfront, but luckily the guy honored the deal and paid up on time, mostly i guess because i didn't hand over control.
i had another person want one of my domains, she agreed to the price but said " can you hold it and i'll pay in 3 months," i gave her no access . nothing ever happened, what i learnt from that was ........... get a deposit....... ANY deposit........ even if its $50 you end up with a beer and a commitment and if they don't come through......no refunds.
if you buy a ticket to the mcg and don't show up...... TOUGH
anyway, i've said enough about a subject i don't often deal with LOL.
hope some of my experiences help.
tim