I would if I needed to, but I would be very very picky. My preference would go:
New high quality seat
Second hand decent seat from someone I trust
New cheaper but OK seat
Second hand decent seat from a stranger
New absolute cheap crap seat
You absolutely need to be aware of the risks inherent with doing this.
It's true that most people would not pass on a crashed seat in bad faith, but after even quite a bad accident most car seats don't show obvious signs of damage. I've heard of people getting the replacement seat through insurance and then thinking Oh I bet this one is absolutely fine, might as well sell it on. I'm sure that's rare but it must be tempting especially if you're struggling for cash and it was a good brand. These were specialist, ERF seats as well - the presumption is often that people who've cared enough about car seat safety to research and source an ERF seat would never do this yet that's exactly who has done it in the examples I'm aware of.
You also have to take into account:
People not knowing that you are supposed to replace them after a crash, not thinking to mention it and then reflexively lying when put on the spot (if you even ask).
People thinking it doesn't count if the child wasn't in the seat, or the impact was on the other side of the car (I've heard both of these).
People thinking it doesn't count if it's under a certain speed.
People in general having a different perception of "accident" to you.
People thinking that the idea of replacing it is just ridiculous overcautiousness and of course it will be fine since it looks fine, not mentioning it in case you the buyer is "overcautious" and it puts you off.
Someone having bought or received the seat second hand themselves and not being aware that it was crashed by the previous owner.
Then there are other issues with second hand seats, not just the crash issue.
There are loads of old, outdated models on Marketplace. People keep car seats for decades and then suddenly decide they need a clear out. You might not at first glance know the difference between a couple of years old model vs a 20 year old model, especially if you're not familiar with car seat innovation and developments. Whereas you 100% know that a new seat conforms to current safety regulations and there are improvements in designs, regulations made all the time. It's a good idea to be familiar with a model you're buying second hand and work out whether it's still on sale and if not, why/how long ago it stopped being sold. Also check for recalls with that model - this isn't that easy to do in the UK as there is no central database.
They might be old in terms of wear and tear. People are not always honest about the amount of use something has had or may not remember, or it might have been passed on to them originally and they don't know. Car seats aren't designed to keep being used again and again for 10+ years - the components like the safety harness will wear down with repeated use and affect the effectiveness. Dirt and debris can get into the mechanisms/moving parts and prevent them working properly. Long periods with lack of use can also affect mechanical parts. There is probably a maximum number of times the cover should be washed before it will start to lose its shape and the cover usually does form part of the safety features. Older children climbing in and out of the seat can damage the polystyrene parts. Baby seats that can be carried tend to get a bit bashed about in normal use. In the EU/UK car seats don't expire, the regulations do instead, but it's not really a good idea to keep using them past about 10-12 years, max. Some people say more like 5-7 years - I think that's a bit overly conservative, but it probably is best to err on the side of caution, especially if you plan to continue using the seat for several more years yourself.
I don't personally buy the "materials break down after 5 years" claim - I think that's highly unlikely - maybe there's truth in it. IMO there are enough good reasons not to use a seat that is too old.
Second hand seats have a history, pieces get lost or damaged over time, parts like strap covers, inserts for younger children, the part to convert it from one stage to another, also pieces can be removed for instance when cleaning or converting from one stage to another, and put back into place wrong. This can be highly dangerous if you don't notice. I've seen seats on marketplace with the straps completely routed into the wrong slots. That kind of thing is obvious, so you'd probably see, but I bet there are instances where it looks absolutely fine but something is not in the right place and that could cause the seat to fail or perform differently to intended. People often own multiple car seats and pushchairs at the same time, and it's common for accessories to get mixed up between them - you often see one seat being sold with a completely different seat's insert because it's been a while since they used it and they have forgotten which goes with which. Someone may have attempted a DIY repair, compromising integrity of parts, or parts may be replaced with third party versions, which is not a good idea as even seemingly inconsequential things such as the cover or pads for the straps have built in safety features and form part of the design. It's quite rare to get an instruction manual with a second hand seat. That's not necessarily the end of the world, you can usually download them, but if you do not know that you can do this then the risk of installing or using the seat wrongly is increased and that is a risk to your child.
If you are looking at a second hand seat, this is what I'd do:
Know the seat, know the brand, know the model. Know what it's supposed to come with and what it's supposed to look like.
Inspect it carefully for signs of fraying, stress marks, hairline cracks, misshaping on the cover. Test all the moving parts (spin, headrest, tethers, 5 point harness).
Think about how long/often you plan to use vs the given age/condition of the seat. (Wear and tear).
Compare with a cheaper new model.
Don't be afraid to ask questions of the seller - be honest with yourself and if you're likely to avoid voicing a concern for fear of upsetting someone, maybe don't buy second hand.
See if the person will let you try the seat in your car.
Download the manual, watch the fitting videos.