Three Pontypridd shops bought by the Welsh Government have been sold on for about £980,000 less than the purchase price.
The units were bought in 2008 for £1.25m as part of regeneration plans.
But proposals for a new shopping centre never came to fruition, and this year they were sold for £271,000.
Plaid Cymru AM Neil McEvoy said the sales were "staggering", but the Welsh Government said they were done on professional advice.
Mr McEvoy said he had referred the matter to the Auditor General Huw Vaughan Thomas, with the Welsh Audit Office saying the concerns were "being considered" and it would "deem if further investigation is required".
Details of the transactions came to light following a request under the Freedom of Information Act by Mr McEvoy's office.
The Welsh Government bought one shop unit at 54/55 Taff Street for £800,000 and another unit, comprised of two shops, at 69 and 69a Taff Street for £450,000.
Both properties were leased - 54/55 until January 2012 and 69 and 69a to July 2014 - with a combined annual rent of £64,500.
But in May the former property was sold for £150,000 - £650,000 less than purchased - while the latter was taken off the government's hands for £121,000 - £329,000 less than was originally paid.
The Welsh Government said the shops were bought in 2008 as part of a proposed regeneration scheme.
"The acquisitions took place near the height of the property market," a spokeswoman said.
"Both properties were tenanted and planning consent had recently been granted for a major redevelopment scheme."
This is understood to have been the Taff Vale Shopping Centre, which was not built.
"Regrettably, as a result of the financial downturn, the proposed redevelopment did not take place and the properties were sold earlier this year following professional third-party property advice," the spokeswoman added.
The Welsh Government later added that the shops "had been identified as surplus stock and we proceeded to sell them".
An assessment by Alder King property consultations for the Welsh Government said the two shops at 69 and 69a attracted a total rent of £28,500 a year, while 54/55 was let for £36,000 a year.
Alder King blamed "market conditions" as the primary reason for the difference in sale price, saying retail investment markets in south Wales, outside of Cardiff, had been slow to recover from the downturn.
Other factors cited included the vacancy of the properties by the time of the sale, as well as the fact the Taff Vale Shopping Centre did not proceed.
Mr McEvoy, Plaid Cymru AM for South Wales Central, said: "The losses the Welsh Government has made on these two properties is truly staggering."
He said this and other examples, including £3.4m economic aid for a steel firm that went bust and the £21m sale of land thought by auditors to have been undervalued by £15m, cannot "be considered as individual mistakes".
A Labour assembly source pointed out that the party was in coalition with Plaid Cymru at the time the decision was taken to buy the properties in Pontypridd.
"Not for the first time Neil has scored an own goal criticising these purchases," the source claimed.
"A Plaid minister was in charge when decision was made", the source added, referring to Ieuan Wyn Jones, who was deputy first minister and economy minister at the time.
In response, Mr McEvoy said he was criticising the Welsh Government's sale of the sites, not their purchase.
A Plaid spokeswoman said: "Plaid Cymru in government did some excellent work in regenerating town centres in places like Pontypridd.
"Serious questions now have to be asked about the Labour government's decision to sell these properties at such a loss."