item sold and it doesn't fit buyers buggy

[PJsAndWine]

I sold a buggy accessory last week which the buyer received today

Prior to buying the item the buyer messaged to ask if it fitted the pram she had. I replied stating I used it on my model of pushchair and that I knew some of the parts were interchangeable between the 2 different models so she purchased it

Receive a message today asking what I'm going to do as it doesn't fit her model

What's the best action to take?

If you read back over your exact wording does it sound like you are saying it will work with hers?

If you said " it works with mine, but not sure about your model" then I'd say onus was on her to check before buying.
If it sounded like you said it would work then I think you'll have to accept a return.

I re-read the messages and I said that the o know some of the parts are interchangeable so it could fit. Then her reply was she wouldn't buy it then the next thing she had

Sounds like it's her problem, not yours. You didn't say it would fit her buggy, so can't be held responsible.

I think it's her issue. You didn't say 'yes, this will fit.' She can re-list it on eBay and sell it on.

It's tough on her part, unfortunately. She can resell it.

Your answer about whether it will fit her buggy does sound a bit vague TBH. It's up to you if you want to refund upon return at this point, but if she opens a bit as described case then she will win and you will have to refund AND pay for the return postage.

Maybe better to offer a refund upon return now, if she pays for the return postage costs. That way you're also reducing the risk of negative feedback too.