IS-28B2 Twin Lark - Service Life Extension Program

Perhaps my own experience may be of help. I own a Twin Lark IS28b2 in the UK. I am now flying it again after 6 years downtime because of the 35 yrs. life limit was imposed by the manufacturer ( who subsequently went bust) and EASA ( a bit like your FAA.) The Rumanian Aeroclub took over as Type Certificate Holder because of vested interest, and have EASA approval for selling to owners an inspection programme. This I purchased for about 500 Euro, and I performed the required inspection with my BGA approved engineer.
I was also given a scrap Twin Lark with an old trailer I bought and have kept some parts. The scrappage was on account of corrosion of the mainspar near the airbrake slots in the wings. The glider had been kept outside,tied down for many years. It was 'beyond economic repair'.
Whilst UK weather may be different to yours there is still the possibility of water ingress and local conditions in the wing which would promote similar corrosion.
Unless you hope to retain and fly it I would agree that the best course of action is to sell the glider 'as is' to an interested person who would inspect with 'fingers crossed'. I suspect that unless your Twin Lark has been tied down in desert conditions it may well have similar corrosion and be suitable for spare parts use only. The other alternative is to arrange for the critical mainspar ( and tail boom )inspection as per airworthiness directives and take a gamble with the money this would cost.
I would add to the sensible advice, on balance, to sell 'as is' and move on..
Best wishes. Pete