It depends on the species. If it's Koi then places like extremekoi forums, or a local club members will often take fish in. The issue is that any taker needs to have the capability to cope with any diseases. Don't dump them in the local rivers. The environment agency will file charges and fines against you given the fish vs native species and the disease issue.
Once sold, you can drain the pond - that water should go down the waste water and not the surface water (again disease risk). Then there's the crap at the bottom - that can go into the bin/tip along with the liner (if it's a rubber liner).
If the pond has a concrete base and fibreglass liner - you'll probably want to put drainage holes in it before filling it. If it's like mine then it's got almost 10" of C35 concrete with reinforcing 10mm mesh sheets at the bottom.
Sell the components - filters, pumps, UV, etc and if there's any pipework - you can probably sell the valves too. When you cut the pipe work - leave some to attach or to help remove it from the solvent weld by the new owner. A 2" ball valve is a good £30-40, a good 110mm ball valve you're looking at over £100 for example. Even multi-bay filters are good.
The cost of sand and filling the hole - is the existing owner going to pay for that? You'll be surprised by the amount of soil required to fill a hole.
