Your gain should roughly be the sales price less the purchase price less any depreciation taken (not including the costs of buying or selling)
If you paid 425K total and you got 175K + 115K +35K or 325K total then the only way you would end with 150K of gain would be if you took 250K of depreciation.
That could be possible if you held the assets for most of their useful life or if you used section 179 expensing to take the depreciation deduction in the year the asset was placed into service.
An LLC can be taxed as a sole proprietorship, partnership, corporation or S corporation. If you have been getting a Schedule K-1 from the LLC then you have likely included the depreciation to reduce your gain in prior years or even to pass through losses to you. Likewise as a sole proprietor you wold already have taken the benefit of the depreciation deduction.
In that case you are correct that there is not any cash realized on the sale of assets encumbered with debt but you do have to include the gain in your income when you dispose assets for which you took an earlier deduction.
It is not clear why you said "I have no way to come up with the money that the IRS says I owe.." You do need to be sure that the returns are (or have been) prepared to get the least legal tax liability and then proceed from there to consider Payment Plans, Installment Agreements if you are unable to pay the tax in full at this time or perhaps even an Offer in Compromise (usually only best if you do not have significant assets as compare to the amount of tax liability)
The Internal Revenue Service will accept an offer in compromise when it is unlikely that the tax liability can be collected in full and the amount offered reasonably reflects collection potential. An offer in compromise is a legitimate alternative to declaring a case currently not collectible or to a protracted installment agreement.
I hope this helps to clarify for you.