Yes, you have to report the sale on your U.S. tax return. The portion of the proceeds that will be taxable is the gain, or profit. The gain is the selling price minus your basis. Your basis is the amount that you paid for the apartment plus the cost of any improvements (but not repairs). Since you have owned it for more than a year it will be taxed as long-term capital gain, which is taxed at lower rates than ordinary income. You can't just report the amount of the gain. On your tax return you have to show the purchase date, the basis, the date of the sale, and the proceeds. All amounts have to be reported in U.S. dollars. You have to convert the foreign currency to U.S. dollars at the exchange rate that was in effect at the time of each transaction.
If it turns out that you have a loss instead of a gain, you still have to report the sale, but you cannot claim a loss on property that was for personal use.
"no receipts kept" - That's obviously going to be a problem. You might not be able to add the cost of improvements to your basis if you have no records of the cost. The lower basis will make your taxable gain higher, so you will pay more tax.
You might also have to pay tax on the sale in the country where the apartment is located. If so, there is probably a tax treaty between the U.S. and the other country that you would have to take into account. You might also be able to claim the foreign tax credit on your U.S. tax return.
In addition to reporting the sale on your Form 1040 income tax return, depending on the amount there might be a separate reporting requirement, but not additional tax, on transferring the money to your U.S. account.
I suggest that you consult a U.S. tax professional about all of this. If possible, look for a tax pro who is also familiar with the taxes in the country where the apartment is located.
