Since you are a U.S. citizen, your sale of property anywhere in the world is subject to U.S. tax. Your gain will computed just the same as it would if the property were located in the U.S. You take your amount realized (sales price less certain selling expenses) and subtract from that your adjusted basis (what you paid for it with certain adjustments). That difference is your gain. IRS publication 551 explains in much more detail how to compute your basis, and publication 544 discusses how to compute and report the gain.
The U.S. does have a tax treaty with Romania, but it will not be of a lot of help to you because the U.S. puts a "savings clause" in all its tax treaties that reserves the right to tax U.S. citizens and corporations notwithstanding anything else in the treaty. Romania will have the right to tax the gain on the property, too. If it does, however, you will be eligible to claim a foreign tax credit on your U.S. tax return for the amount you paid. If the Romanian tax is larger than the U.S. tax, then the effect of the credit will be that you pay no U.S. tax on the gain.
If you have the funds directly wired from a Romanian bank into your U.S. bank account, you should have no special reporting requirements. Instead, the bank will report it if needed.
This is complex enough that you may want to see a local tax professional who is familiar with these sorts of issues.
You can get forms and publications at most IRS offices, have them mailed to you by calling the IRS toll free at 800-829-3676 (800-TAX-FORM), or download them from the IRS at: