We had a positive experience with a part-exchange recently with a major house builder, if it's any help.
We'd been trying to sell DP's old flat for 18 months on the open market but with no luck - we'd had offers but on both occasions, the potential buyers later turned out to be unproceedable (due to misrepresenting their own situations and issues with their own properties). All very disheartening & upsetting.
We then found a new build estate that we loved and got all excited when they mentioned that they could do a part-exchange on the plot that we liked. There were various criteria (e.g. existing property could be worth a maximum of 1/3 of the new property purchase price & if leasehold, had to have at least 99 years remaining) but thankfully the flat qualified. The builder reserved the plot whilst the valuation of the flat took place.
The valuation took just over a week and essentially they asked two local agents for 'quick fire sale values' - this means how much they would have to advertised the property for in order to achieve a sale within 6 weeks.
Helpfully, the two agents that they used were fairly generous in their valuations and the offer that the builder came back with was equivalent to the offers we had accepted previously via the open market. However, there were of course no Estate Agent fees to pay so we were actually better off financially, not to mention better off in terms of peace of mind!
One downside that we found is that the builder's solicitors were shit hot - far better than ours. Everything and anything was queried, particularly as the flat was leasehold. Thankfully as we were directors of the flat management company, we were in a great position to answer all queries and to know that everything was above board. There were still several really silly points raised about irrelevant stuff and we had to buy bespoke indemnity policies to appease the builders; had our solicitor been competent, this may not have been necessary. We did feel that we had a weaker bargaining position due to the part-ex element.
As the onward sale of the flat completed within 6 weeks of our completion date, we actually got a 'commission payment' of £500 a few weeks after we moved in. This was v helpful and went towards our fitted wardrobes 