Depends on what brand you are looking for, your budget, if you care about having a warranty or not, etc. Also are you looking for a Grid-Tie Inverter? Hybrid? Standalone?
Lots of solar companies go bankrupt, so I've seen a lot of brand new, no warranty, legit, UL1741 GTI on ebay for like 80% off (Buy 2, it's like having a warranty?).
Keep in mind that the better the brand, and the more in demand the inverter is, the harder it will be to get a deal on it.
For off grid, Aims is probably the most used American owned/Chinese made "budget" inverter. If you are looking at that, keep an eye on Amazon Warehouse for deals, although they are rare now that Amazon no longer sells those inverters directly. For brand new, take a look at Zoro, their regular price is laughable, but they have 25% off sales often (had a flash sale today), which makes them cheaper than the other inverter suppliers and you still get the full Aims warranty.