Here is what I want to do
Mrs Trout wants to try biking again so the plan was to get a MMMbop frame the same size as mine and then build one up for her .

nothing left in the parts bin so thought about looking for a hardtail at a good price for the donor parts then selling the frame afterwards.

would this work out cheaper than buying the parts seperately

Sometimes it does. I bought a seat and post from a guy local to me off ebay. He had bought a complete bike fairly cheap, robbed the parts off that he wanted and flogged the rest. He reckoned it cost him almost nothing by the time he had built and flogged.
A couple of other people have done the same. Just depends on how quick you want to build a bike. If you can wait until a bargain comes along for the parts, then thats better. Mind you, there are some new bargains out there from Merlin, On One etc.

Mrs Trout wants to try biking again so the plan was to get a MMMbop frame the same size as mine and then build one up for her .

Sound idea, except is she really the same size as you – all directions?

Probably better with a Lady specific – unless of course…

I have a bigger belly but yes we take the same sized bikes .
both 5 ft 9 inche tall though she has longer legs than me .

Seens as she's just 'trying' biking again, whay don't you just buy a decent spec hardtail and then if she gets into to it flog the frame adn build exactly wheat she desires

I know this is going to be a short lived trial so when she stops riding it at least I will have another bike to ride or sell .
and am also being underhand in getting some upgrades for my bike here and this is an ideal chance to swap out my older bits 😉

Since you're not bothered about the frame size of the donor bike,you should be able to pick up a good bargain on a 2010 model. Just try not to be too picky about the specification.

Last summer I did some spread sheets on building up a bike. I think the answer is its tough to call and its only reall cheaper to go down the doner bike route if you find an amazing deal and your really going to use every thing.

If you start swapping stem, seat post and replacing cables it eats into the advantage

But buying a lightly used women's specific bike would be the cheapest option it won't loose much value if you need to re sell it and it'll save alot of farting around.

I'll venture an opnion that's not been asked and is probably unwanted.

Either buy yourself another bike or buy one for Mrs Trout. I don't think either project will benefit from trying to be the other……

The hardest part is finding a donor bike with long enough good forks.
I've been following ebay Marin Rocky Ridges as a good mmmbop donor bike, but I think I've given up on the idea for now.

buy a used bike of the right size, then flog it at no or minimal loss if she doesnt get into it?